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.
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.
To benefit from the full power ouput of the A90 you should use a suitable balanced source along with the balanced output into your headphones. The gain settings and low noise floor, along with low output impedance mean the A90 is also very capable when using IEM's, just be careful with all that power on tap.
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.
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)