I've not had my A90 for long, but it's obvious straight out of the box that it's an amp that sounds great, not just one that measures well. No surprise there, of course but it's especially good for the asking price. Design and build quality are very impressive.
Four subjective first impressions from me:
1) Great features, front switches and balanced in/outs (so far I've only used SE output). Runs only warm.
2) Very clean, neutral, dynamic sound, as measurements would suggest. Well controlled bass.
3) Pairs really well with my HD800. Perhaps not quite as smooth & open sounding as HD800+Trilogy 931, but impressively close.
4) Good sound with DT880 600Ω. Usually enough volume using high gain, though I am at >75% volume for some classical. The A90+DT880 sound is possibly a little flat compared with amps that can provide more power. I might try to provide the A90 with a 3V input to see if that helps it pair better with the DT880s.
I am exceptionally happy with the A90. It will displace others in my collection. It's a stunningly good amp for nearly all of my headphones.
If I didn't love the sound of the 880s when driven with 600mW potential, I now probably wouldn't need any other headphone amp. So for most people, this could be an 'end game' amp. I am now likely to order the D90 to pair with the A90, so I'll get remote volume control.
This headphone amp will embarrass many far more expensive amps.
(I used: QobuzHD/FLAC->Roon->USB->Chord Qutest @ 2V/SMSL Sanskrit 10th Mk2->A90 SE->54yr-old ears. Classical, Indie & Hard rock music.)