I've owned the AD2kx for about 3 months now.It was slightly lacking in treble, compared to my AD900x but it is no longer of my concern. I deeply appreciate how close-sounding this headphone is (it's definitely not a volume bias), yet still provide a fulfilling soundstage, so you don't feel claustrophobic. My A900x and my Westone UM3x are example of cans that felt claustrophobic to me. The AD2000x is now my main headphone instead of my A900x.
I couldn't resist typing out about the A2000x too. Once you have a proper seal (A2000x craves it BADLY. Refer to the A2000x thread for proper way to make it seal), I think the A2000x has no equal in the closed-cans area and could pretty much go head-to-head with many open cans. The clarity in this can and the soundstage it has, is just amazing. I didn't think the soundstage could come from a closed can. I won't say that the AD2000x is better than the A2000x or otherwise, as their sound signature is different, albeit still mid-centric. AD2000x just gets my frequent use because I need to hear people calling me in the house, and my A2000x is my office can. The A2000x pad material feels more comfortable to me, but it gets hotter compared to the AD2000x.
Also, in case anyone needs this info: Since the higher end AT cans and all the AIR series don't have that swivel that the A900x has, getting a proper seal is hard. A proper seal won't hurt your jaw, and if it does, you're doing it wrong. You have to change the angle where the headphone sits at. This is the one terrible drawback of AT cans, they're apparently tailor made to the current long-term user. Anyone that wishes to audition it will have to cup the cans hard.
I'm quite satisfied with my AD2000x and my A2000x. I'm willing to drown in ignorance about any better cans out there and say that these are the best cans in existence. I really felt that I've reached audio nirvana with these cans.
If there's any closed cans that can come close to the A2000x in soundstage and SQ, or any open cans that can come close to AD2000x extremely close-sounding soundstage, I think I'll find a place to audition them. Other cans are apparently easier to find for auditioning in my area. They sell AT cans here too, but they don't really put them up for auditioning, strangely.