I’ve had my AD900x for about 3 years now, no issues. I was not thrilled by how flimsy it felt back then but… well… I’m a fan of Koss products now so what’s there to complain? If anything, it’s very comfortable as long as you don’t run into issues with the circular earcups.
Sound-wise, I think my AD900x is pretty nice. Midrange forwardness, a very de-emphasized 4-5khz range which looks ugly on paper but in practice is nice for the lack of sibilance, and emphasized treble extension. The sound signature is far from flat, but I do like them for soprano, denpa, and other higher-pitched signing. Some people do find the AD line a bit too shouty, but I really do prefer such forwardness. Tuning does make sound seem pretty clear. Bass is tight, treble is sparkly, etc. It’s not clarity like an 95X does through technicality, but still enjoyable.
Tonality overall - 1=recessed 2=natural 3=emphasized
mids and lower treble=3