After going AD900x to AD1000x to AD2000x I decided to buy the AD900x to see the differences as I remember liking them quite a bit at the time (currently own 2kx). First thing that strikes me is they are definitely more comfy, the clamping is just right and the drivers don't rest on my ears. Comfiest phones for me, and recently I've owned HD800, q701, Shure 1540 among others.
Soundstage is bigger because the driver isn't so close to your ear. AD900x are really a bargain, I'm actually debating which I like more at this point, certainly not a world of difference. There is more treble than the 2Kx which I like.
Interesting impressions! I haven't tried the big brothers to the AD900X, but I still really love them after a few years and feel no need to part with them. They are my favorite headphones for watching movies and concert DVD's etc. And as you said, they are ridiculously comfortable. Long live the AD900X's!