TOPPING A90 Amp Reviews Compendium

Compendium updated February 25th, 2021.

The popularity of our SMSL SU-9 Compendium made us decide to go through the existing reviews for our most popular products and do the same thing.

Below you will find a collection reviewing the TOPPING A90 Amp. While many of you have already read these reviews in their entirety already, we've combined and collated the material all in one spot for you to easily go back and find the highlights to help you make a purchase decision. If there's another product out there that you would like us to do a compendium on, feel free to let us know! In the meantime, enjoy the compendium on the TOPPING A90 Amp!

 

Audio Science Review's article on the TOPPING A90 Amp, originally published on May 25, 2020:

“The A90 is one of the best executions of the Topping series of products.

Headphone Amplifier Audio Measurements

SINAD which represents the worst case/sum of noise and distortion is at 121 dB. Distortion output is less than -140 dB. I put the analyzer in loopback mode measuring itself and distortion only went down a to -150 dB. So we are not far from my state of the art, nearly $30,000 analyzer utilizing dual analog to digital converters to achieve this!...

Here is the ranking in our headphone amplifier table:

If distortion and noise are inaudible, let's see if there are any frequency response variations:

Dropping the output down to just 50 millivolts to determine what dynamic range we have for very sensitive headphones the A90 takes the top spot:

There is none well beyond our hearing range. This proves yet again that you have superb transparency here.

We have a bit more power and seemingly equal distortion at at the limit…. My threshold of adequate power in this test is 100 milliwatts and we have double that. So headphone users like Sennheiser HD650 should be very happy as should people with even higher output impedances.

Headphone Listening Tests

I started my testing with driving my very inefficient and low impedance (25 ohm) Ether CX headphones. These headphones bring most headphone amplifiers to their knees with either distortion, lack of power, muting, or sum of the above. None was in play here. The A90 drove them with authority using its balanced XLR output to ear bleeding levels! It was the best I have heard them sound. Detail, bass impact, loudness, you name it, it was there.

Switching to the Sennheiser HD-650 repeated the same experience even though I opted to use the 1/4 unbalanced out. I was sitting there with smile playing track after track.

Conclusions

The Topping A90 comes later to the party of leading edge headphone amplifiers. It comes there though with guns blazing producing exceptionally low noise and distortion while providing ample power to drive just about any headphone. It has an elegant enclosure which is of course a match for their excellent D90 DAC. The combination should be beautiful to look at, as much as listen to.

I am happy to give one of my strongest recommendations to Topping A90.

I live for days like this....

 

Headphonia’s review of the TOPPING A90 Amp, originally published on June 6, 2020:

“Well, it seems that the good people at Topping have certainly made an earnest tilt at what seems, on paper, to be a pretty ‘perfect’ kinda headphone amplifier: the new $499 A90.

The A90 is a full-sized desktop headphone amplifier and preamplifier, with fully-balanced circuitry…. IEM users might bemoan the lack of a 2.5mm balanced or 3.5mm single-ended connection, but Topping’s choice of 4.4mm Pentaconn is a statement of intent that the A90 is intended to be a future-facing piece of equipment, and also one that the owner can hang onto for a long, long, time in terms of its power and versatility. 

Speaking of power, Topping has squeezed a shed-load of it into the A90. It’s capable of a whopping 7.6 Watts of power into 16-ohms through either of its balanced connections and still manages to eke-out a whole Watt when driving a higher impedance 300-ohm load. The A90’s is no slouch in terms of its single-ended performance, being capable of a healthy 2 Watts at the usual 32-ohm benchmark. 

Presentation, design, and build.

I was actually surprised by how compact the A90 is. When you think ‘Flagship’ and ‘Desktop’, you’d be forgiven for thinking this might take up half your table-top, but the A90 is about as large as a medium-sized paperback novel. Or, 22cm x 16cm x 4.5cm to be precise, with a weight of 2.5kg…. It sits on four small rubber feet, and you are often reminded of just how slight it is when plugging/unplugging headphones or swapping cables, as it does tend to slide around a bit. 

Desktop and system setup. 

The A90 is a fairly simple analogue instrument and requires very little learning to live with – it’s fairly straightforward and intuitive to use. I did scratch my head once or twice wondering why no sound was coming out when I forgot to select the correct ‘RCA’ or ‘XLR’ line-level input, but that’s about it. It’s more of a headphone amplifier with preamp functionality than vice versa….

If the shape and model name of this amp rings a bell, it might be because you’ve already come across a review of the A90’s digital stablemate, the fully balanced D90. Topping designed the A90 and D90 to work together as high-end counterparts to one another to create the ‘Top’ Topping stack. 

Performance and Sound Quality

It can be a little intangible when trying to impart the nominal differences between the sonic qualities of excellently implemented solid-state amplifiers, but the Topping A90 impresses immediately straight out of the box. In terms of its sonic characteristic, the A90 an utterly transparent, dynamic, and uncoloured presentation.

It’s terrific to have the ability to use the A90’s fully balanced configuration, utilising the superior noise-rejection of XLR cables (especially over long cable runs), and to take full advantage of the A90’s power via the balanced headphone outs. However, the A90 also happened to perform perfectly-well with both single-ended inputs via the RCA-in, and via the single-ended headphone-out when tested. 

The A90 presents no hint of peak or dip at any frequency – it’s an absolutely achromatic playback experience, yet it isn’t at all devoid of musical enjoyment. Some cheaper solid-state amplifiers can tend to add a sense of ‘glare’ or sterileness to the treble department, but nope – nothing to report here….The Topping A90 provides a flawless peek into all the details of a piece of music, good or bad. If it’s there, it will reveal it. If the transducers of your headphones are capable of reproducing it, it’ll do it for them. 

Conclusions

The Topping A90 is an absolute no-brainer recommendation as a balance solid-state amplifier. At $499 its performance will embarrass far more expensive offerings, and if you’re looking for future-proof power, connectivity and usability then there isn’t another option under $500 that comes even close – especially if you need pre-amp facilities.

Consider me impressed with the A90’s build, overall quality and excellent form-factor….The A90 simply removes itself from the playback experience and provides absolute fidelity from the musical source directly to your transducers. It’ll give you the truth, and then some.

 

Soundnews’ review of the TOPPING A90 Amp, originally published on June 14, 2020:

There are a lot of things to love about A90 and very few to nitpick. Don’t you worry, every pros and cons will be mentioned in this review, so let’s carry on and unbox this little big amplifier.

Inside the Box

Since A90 has the exact same dimensions of the case as their D90/D90 MQA units, Topping used the same thick card-board box with lots of foam inside it for protection. Inside you’ll find a power cable, a 6.35mm to 3.5mm headphone adapter so you can use portable headphones with it as well, there is a warranty card and a super-detailed user manual. 

Design & Build Quality

I really liked the look of their DX7 PRO, D90 and D90 MQA and A90 is really no different to those units. It has the same build quality and the same small and simple form factor. It has a small footprint so you’ll have zero issues inserting it in a speaker or headphone setup. The case and the front panel have rounded edges and corners and are quite pleasant to the touch. 

There are lots of small details that I’m really liking on this case, for example there isn’t any visible screw on all its sides, giving an impression of a classy handsome look. Probably one the best things to know is that it uses a unibody case with a simple front and back-plate attached to it, so it simply feels really nicely put together. 

Controls & Connectivity

There are several things that I like about A90, it was wise going for the usual stuff as 6.35mm jack and 4-pin XLR jack, but the newest 4.4mm balanced output caught my attention the most – this is currently my favorite connection, its nor too big and nor too small and it could be used for portable and desktop headphones. The front panel is straightforward and very simple to use. On the back you can spot two pairs of analog inputs and outputs coming in RCA and XLR flavors.

Under the hood of A90

The circuitry inside A90 amp is a combination of multiple op-amps that are cascaded together with a feedback loop. Those feedback loops are negative; thus, it is called a nested-feedback amplifier.

So, what is so cool about a composite topology? Well, first of all it exhibits a higher gain, higher slew rates and much lower distortion. A composite op-amp is placed at the input and the output of another op-amp, in this case it was the (in)famous TPA6120A2 that Topping used. The composite op-amp is basically enhancing the performance level of TPA6120A2 with the help of negative feedback.

The good part is that a circuit like this brings the best of both worlds with excellent DC and AC characteristics, the feedback loop will also remove noise within the circuit. The bad part of such designs is that feedback loop reduces the overall gain of the circuit, weakening the signal at the output.

If this sounds like some gibberish non-sense, in simple words - these are really high-end features that can be found in very expensive designs. I don’t want throwing stones at nobody so I’ll stop here but I will just mention that A90 is already proved its value to me.

Sound Performance

I. Preliminary Impressions & Tonality

The biggest compliment I can give to A90 is that I felt like I opened my windows towards music, I simply experienced the cleanest, the most transparent, colorless and effortless experience I’ve had with headphones. A90 is doing that on such a high level that you will be limited by your source and not by the amp as it usually happens. 

When you experience transparency at its purest form, detail at its maximum and the widest bandwidth and at best damping factor, you simply forget about general notions tied to sound quality as frequency response, transparency or transient response. These things don’t matter anymore because they are simply rendered at the best your source and headphones could deliver. A90 feels like listening to the real thing without anything staying in between you and the music.

II. Resolution & Transparency

This is where A90 feels like the most powerful and beautiful muscle car from the parking lot, it’s simply a head turner, catching attention of everyone passing by. When it comes to resolution and transparency, A90 together with HPA4 are playing this game at the highest level possible. Forget Drop THX-AAA-789, forget Monoprice THX-887, forget SMSL SP200, Topping A90 is still playing that at a higher level than any of those and the only real contender is only the Benchmark HPA4. I’m sad and super pleased at the same time, sad because I still paid $3000 for my Benchmark HPA4, but super pumped and very happy for all the folks that can experience true high-end headphone listening is at only $500, there isn’t a better time to be a headphone listener. 

A90 is simply the definition of an invisible headphone amplifier, it simply takes a step sideways and lets the music flow directly to your ears, without adding or taking away anything from the purity of the musical signal.

A90 is as achromatic (colorless) and as linear and as extended as headphone amplifiers could go and, in this regard, I am placing it at the highest level together with the Benchmark HPA4 as the true rulers of the headphone kingdom when it comes to solid state amplification. 

III. Noise Floor

The key ingredient of unlocking a see-through transparency and the highest level of detail retrieval is having the lowest noise floor possible at any volume level….Topping focused a lot of their efforts in lowering it as much as possible. The fruits of their labor speak for themselves as A90 has a maximum noise level of 1.3 microVolts – this is really world class performance and puts it in the same boat with Benchmark HPA4 and Sparkos Labs Aries! 

Without thinking too much, I connected the most sensitive IEMs at my disposal and those were the FiiO FA9 and FH7 IEMs. I tried them on the 6.35 mm jack first, I tried two software players and also Tidal desktop app. Volume wise the highest I could go on low-gain was about 12 O’clock, about 10 O’clock on the mid-gain and about 9 o’clock on the high-gain. The most amazing part? If I would pause my music and go all the way up to maximum volume, no matter the selected gain, the noise floor would be simply undetectable! I’ve heard absolutely nothing; it has simply a noiseless and the blackest background I’ve experienced. I’m very glad to report that A90 works absolutely amazing with IEMs!

In terms of noise floor, I experienced few close to perfect designs, but the best ones were always the HPA4 and Sparkos Aries and I’m happy to add the A90 to that list as well.

IV. Transient Response

A90 is what I call a sleeper headphone amp - it’s quite small that plays in the lightweight category, but when you press play, an unstoppable force will start pounding your ears with increased dynamics. 

Funny fact is that it can even transform some of the lazier sounding headphones like Sennheiser Momentum 2 or Meze 99 Classics, it simple enhances the speed of their membranes and lowers the decay making them sound almost like planar-magnetic headphones. The output power plays a bit role here, those 7.6 Watts are adding a lot in terms of dynamics and diaphragm control.  

A90 still can’t pound as hard as Flux Lab Acoustics FA-10 is doing, but it follows its footsteps together with HPA4 and Sparkos Aries. FA-10 is simply a different (untamable) type of beast that can’t control its temper. By comparison, A90 has a calmer personality, yet if you are calling for the thunder, thunder you will get!

V. Soundstage & Depth

A90 is having a very clean and precise leading edge, a very clean outline of every note, you simply feel its body and its borders, you feel it carrying weight and impact....There is no need in giving examples from my music collection, if the record was made in a bigger hall, you will feel that, the borders to which the sounds could fly are not felt. When moving to cozier recordings made in smaller rooms, the stage would decrease in size and you would feel those artists much closer to each other.

I want to point out that A90 will not artificially boost the soundstage size how some amplifiers are doing (mostly tube-based amplifiers) and will render soundstage size as close as possible to the original intent of the mastering engineer. If the record was intended to sound big, wide, tall and deep, then by any means A90 will sound like that and vice-versa, closed-in records will be felt smaller in size to a point of being claustrophobic.

With all that said, I want to point out that while A90 has a great soundstage size on all X, Y and Z axes, this is probably the only area where I feel it is not outperforming the Benchmark HPA4 and Sparkos Labs Aries.

VI. Frequency Response

This will be a very relaxed chapter because A90 is the definition of linearity and by that, I mean simply a perfect frequency response, from the lowest pits of sub-bass information to highest levels of treble, A90 rendered all that without flexing a muscle.

Sub-bass goes to the lowest octaves, it is also incredibly clean, detailed and super tight sounding.

Midrange had the right tone, voices felt real and had the right pitch, there was naturalness in my old smoky jazz recordings, I felt the raw energy on my older rock tunes.

Treble goes high into the sky and can’t be stopped. It is very defined but also really extended in the top octave that improves the performance of the Topping D90 and D90 MQA DACs.

VII. Power Output

Audeze LCD-4 and Hifiman Arya are two of the hardest to drive headphones I have tested up to this point, LCD-4 is particularly picky when it comes to power as I have experience them lazy sounding with few particular amps. 

As you can imagine, A90 has gobs of power and should comfortably drive any headphone from your stable, including the notorious Hifiman Susvara, HE-6 or Abys AB-1266 should be driven at their fullest. A90 is really one of the most powerful headphone amps I had the pleasure of testing, it actually outperforms a bit the Benchmark HPA4 and quite a lot more the Sparkos Labs Aries in terms of pure raw power. As small as it looks, power wise it is actually on the same level with the Burson Conductor 3 Reference and very close to that monstrous 15-kilo Audio-GD Master 9 amp. Flux Lab Acoustics FA-10 still reigns supreme, with its 16 Watts of power you can make your headphones sound as speakers if you want.

Conclusion

It should be pretty clear by now how awesome the Topping A90 is in a headphone setup.

It caters not only to desktop headphone users by offering some juicy 7.6 watts of power, it also caters to the IEMs users but offering one of the lowest noise floor I have ever experienced. Everything I have said about the Benchmark HPA4 in its review can be said about the A90 as well, it is really that good. Considering the price point that Topping went with unit, I’m proud to announce that A90 receives my highest recommendation as being the one of the most complete sounding amplifier at a very attractive price point! It is also the perfect companion for your freshly acquired Topping D90 or D90 MQA DAC.

 

Sound Perfection Reviews’ article on the TOPPING A90 Amp, originally published on Sept. 3, 2020:

Packaging, Accessories and Build Quality:

The A90 comes in a similar box as the D90, it’s a simple black affair with very good foam padding inside to keep the unit safe during shipping. It really is as simple as it gets, but the box feels sturdy and does the job of being nice to open whilst offering excellent protection during shipping.

Accessory wise the A90 comes with a jack adapter and the power cable. To be fair though the A90 is just a headphone amp so there aren’t really any accessories needed.

The A90 is similar in build to the DX7 Pro and the D90, with smooth edges and a good size chassis for desktop use. There are 3 switches on the front along with 4.4mm and 4-pin XLR balanced outputs, 6.3mm single ended output and a volume knob. On the back you have RCA and XLR inputs and outputs, the power cable socket and a power switch. On my unit everything feels very well put together and solid, the inputs and outputs are all tight and the switches have no play in them.

Functionality:

The A90’s main function is as a headphone amplifier, and there are few frills to the amplifier stage. You get 3 gain settings, a switch for balanced/single ended inputs and a switch for the power (Off/HPA/PRE). There are both balanced inputs and outputs, which allows you to use the A90 as a pre-amp for desktop speakers.

Sound:

Subjectively the A90 is very clean and linear with a hint of warmth. The great thing about the A90 is that you can pretty much power most headphones on the market with it, and it serves as a great amp to listen to but also as a great benchmark. For it’s price range I am really impressed at how detailed and clean the A90 is, it amplifies the sound without altering it or adding much of its own flavour.

Using my GMP 400 which are quite a neutral reference for me, the bass is tight and articulate, the midrange is controlled and balanced, and the highs have tons of sparkle and great extension without sounding hot or too bright. Yes I tend to prefer these with an OTL amp to add a little body, but the fact the A90 can drive them in low gain is really impressive.

Switching to the HE6SE, apart from the V281 I don’t think I’ve had another headphone amp come through that drives them as well as the A90. Medium gain is all that’s needed for my listening levels, and they come across beautifully balanced yet with more than enough impact (I’m used to them being used with a speaker amp). The A90 in my personal opinion drives the HE6SE really well.

Moving to IEM’s on low gain the A90 has a very black background with no noticeable noise, again the clean nature of the amp really shines through here. I have more than enough control over the volume knob without imbalance at low volumes with sensitive IEM’s which is great.

One place where the A90 doesn’t necessarily add anything is the soundstaging, overall the sound is tight and controlled but it’s not what I would call expansive. It doesn’t throw the sound overly wide, but then again it does a great job at just amplifying the source material. There really is only a hint of warmth from the A90, but luckily it steers clear of muddying any of the detail.

Conclusion:

Topping, as I have said before, have been on a bit of a roll lately. The D90 is a top performing DAC at a midrange price, the A90 adds tons of amplifying power with a neutral yet slightly warm sound. The D90/A90 stack will suffice for 99% of headphones on the market, even the most hard to drive, and sounds superb doing so. For reviewers like me, it also serves as a superb reference stack and a great benchmark. Like the D90, I cannot recommend the A90 highly enough for the price point.

Sound Perfection Rating: 10/10 (versatile, tons of power that is cleanly delivered)

 

Audio Discourse’s review of the the TOPPING A90 Amp, originally published on Sept. 6, 2020:

The A90 is a linear solid-state amplifier from Topping out of China. Topping's recent lineup of amplifiers and DACs have received a lot of praise for their ultra low noise measurements and objectively have been found to be in the top echelon of high measuring amplifiers for headphones on the market. 

These impressions won't dive too much more into measurements, but will focus more on my user experience and listening impressions.

User Experience

The Topping A90 has a nice array of switches, knob, and input/output options. It has both RCA and XLR input and output on the back, and an internal power supply unit, so a standard power cable will work fine with it. 

On the front, it has a large volume knob that has a nice balance of smoothness and weight, and blends in nicely with the silver chassis. On the left side are three toggle switches controling the power on/off, headphone amp or preamp function, and one of three gain modes. There is also a power flip switch on the back on the power supply.

For headphone output, there's the standard option of 1/4 inch single-ended amplified output, as well as balanced options of 4-pin XLR and 4.4mm Pentaconn. One of the driving factors for me obtaining this amplifier was for that 4.4mm output as I have grown very fond of this connector and have switched all my IEMs and headphone cables to this connector type.

Now, onto the not so good -- ground noise. 

There's a chance that if you use RCA stereo coaxial inputs as your source into the amp, you may get humming or ground noise. I experienced this when I used my Schiit Saga+ Preamp, but it did not occur with using the Schiit Bifrost 2 DAC when paired with RCA connectors. One thing to also note was that this loud humming noise was really bad when I had plugged my iPhone 5S into the Preamp as a secondary source, even when it wasn't the selected source input. It seemed the noise worsened significantly if it was being charged, and decreased when unplugged from power. That said, the Saga+ going into the A90 still had a ground noise that was audible.

Using XLR input totally kills this problem but it is something to note. Topping/John Yang mentioned that this is probably due to the design choice of using an internal power supply unit instead of an external power brick, however, I will note that I never heard this noise using the Schiit Saga+ connected to either the Schiit Asgard 3 or the Schiit Jotunheim, which also both have internal power supplies.

Sound Impressions

I have owned several THX amplifiers in the past, and have also owned a couple Topping Amp/DAC combos and most recently I've used the RME ADI-2 DAC, Schiit Asgard 3 and Jotunheim, amongst many others for solid-states. They aren't necessarily my primary listening devices though, as I typically am using one of my tube amp setups for general headphone enjoyment. But I still use IEMs via solid-state amplifiers and headphones on them for review purposes.

Of all of them, I believe if there was a meaning for the term sterile in regards to sound, I think the Topping A90 is the sterilest of the the sterile crowd - which I'd lump in the THX amplifiers I've owned -- The Massdrop THX 789, Monoprice THX788, and the SMSL SP200, and to some extent, the RME ADI-2 DAC. 

The A90, outside the humming issues, has a very black background via XLR, and in my opinion, a colder tone. Coming directly from the Schiit Jotunheim, which I thought was a cooler signature than the Asgard 3, I found the Jotunheim to be warmer and bloomier. The A90 has a sharper edgy sound to it, that is a bit more forward sounding, especially in the upper mid-range and treble regions. I actually do like the bass response, as I am one to like quicker transient responses at times, and I feel like this one definitely has a noticeably different type of agility than the Asgard 3 (for sure) as well as the Jotunheim.

I do feel it lacks a little bit of soundstage depth, especially compared to the Jotunheim and my current tube amp setup, the Feliks Audio Elise.

Most of these impressions were using the ZMF Verite and Hifiman Arya headphones out of the balanced outputs, but when it comes with using IEMs, I am quite okay with this more sterile sound for whatever reason. The noisefloor is quite small and the ultra low output impedance is great for IEM usage, especially ones that are sensitive to impedance mismatch and/or have high sensitivity. This is a farcry from the iFi Hip DAC and iFi Zen DAC, which I am demoing and use at work for IEMs (respectively). Both of these have really poor noise floors and audible white noise comes out of the 4.4mm balanced outputs on both units.

Wrap-Up

The Topping A90 is lauded as one of the best measuring headphone amplifiers on the market right now, and I can't deny the numbers that the manufacturer and independent analysis has shown. It also has a clean noise floor under the right circumstances, but for my personal take, I don't gravitate to using this as much as other amplifiers.

This is mostly due to the more fatiguing sound that I am not as keen to. I'm also not super thrilled about the grounding noise out of RCA inputs, and while that's not a deal breaker if you can have a balanced DAC, it can be if you do not. 

But with all that, the convienences of multiple I/O options as well as the addition of 4.4mm balanced headphone output make this a compelling unit with plenty of power to take on all my headphones with ease. The A90 is going to be a bit controversial I'd say, but it will work for me as a solid-state reviewer unit, and one that I will continue to use for that purpose.

 

In-Ear Fidelity’s review of the the TOPPING A90 Amp, originally published on Sept. 6, 2020:

Introduction

So before you proceed, just know that this is less “a review of the Topping A90” and more “rambling about why the Topping A90 is going to be used for future headphone reviews on IEF”.

Topping is a brand that has taken the headphone-source-market by storm, with the ever-growing ubiquity of measurements and the rise of Audio Science Review, arguably the authoritative stronghold for such discussions. For those seeking sources with the highest SINADs and dynamic ranges, Topping is usually the first thing to dominate the conversation and so has been gaining traction within these circles as a “default recommendation”.

The Ground Loop Issue

Unfortunately, I’m a victim of the infamous ground loop issue that had plagued other Topping owners in the past. On the DAC side I use either the iFi iDSD Micro Black Label or the Motu M2, both of which are USB bus-powered when plugged in. Though do note that while the ground loop hum was basically inaudible with open-backed headphones, it was basically unusuable on IEMs (especially when said IEMs were plugged into the 4.4mm balanced outputs).

While both of the M2 and the Black Label were perfectly fine when used by themselves with their own headphone-outs, the problem kicks in when the A90 is connected to them via single-ended RCA. Basically:

• PC > USB > M2 = Clean
• PC > USB > Micro BL = Clean
• PC > USB > M2 > RCA > A90 = Ground loop hum
• PC > USB > Micro BL > RCA > A90 = Ground loop hum
• iPhone > USB > Micro BL (battery) > RCA > A90 = Clean

Fortunately, switching to 1/4″ TRS balanced-out to XLR balanced-in fixes the ground loop. But it’s still a little disappointing that I’m locked out of SE inputs on a $500 amplifier due to noise issues.

Performance/User Experience

The A90 acts as both a headphone amplifier as well as a preamp, which helps my audio chain a ton ever since I got my newly-acquired Neumann KH120s.

Power is not an issue, though my own listening volume requirements are not high in the least. With my DAC (Motu M2) almost-permanently at 50%, I hardly ever go above 50% on the A90 itself. This is with the Sennheiser HD800 and the Hifiman HE1000, both on low gain too.

The output impedance of the A90 is plenty low enough to not cause any trouble with “problematic” IEMs (I’ll defer to ASR for actual non-manufacturer measurements), with single-ended being the most optimal for IEMs. 

Sound-wise, the A90 is as you would expect: a clean, basically uncoloured source that probably wouldn’t be a good fit for many headphones (signature-wise), unless you got yourself a really warm, really blunted headphone (*cough* Nighthawks). There is a bit of sharpness going on in the overall presentation of the sound, though nothing I would call offensive or a dealbreaker.