Like akarise, I too just received my AD2000X from Japan.
I've only had a chance to listen for a few hours, so this is a very preliminary comparison.
First, I compared the AD2000X to the AD2000 on the Pan Am amp using Cifte tubes. Both headphones sounded wonderful on this setup - 9/10 for both. The 2kx is a very different presentation from the 2k. Like akarise said, the 2kx gives a much more "in your face" presentation. It has a closer-in soundstage and more overall edginess and contrast. I perceived this as slightly elevated bass and treble levels, which pushed the mids back just a tad - still very good mids - but not as forward as the lows and highs. The 2k, on the other hand, gives a more even, wider and less "contrasty" rendition and soundstage. It also has a touch less bass presence.
Compared to my 900x, both the 2kx and the 2k are much smoother and clearer across all frequencies with better detail and balance. The 2kx has a "family resemblance" to the 900x (punchier rendition; bass/treble forward) but the 2kx is more sophisticated.
Both the 2kx and 2k sound so good that I am keeping them both. One or the other would be better depending on the music. I may or may not keep the 900x, depending on whether or not I can find a good portable amp match-up (so far, not that great with both the ALO Rx-Mk3-B+ or the Meier Corda Stepdance at 15V).
Also, keep in mind that the 2kx has only had about 3 hours on it - and I heard it open up and get smoother during that brief time period. It may keep improving - stay posted!
Edited by R Scott Ireland - 8/28/13 at 12:54pm