I just got my AD2000's (very lucky to snap a pair up from an Australian member in the FS forum). Ever since getting the AD900's I've wanted to get the 2000's, and it turns out that the difference between them in terms of sound is not as drastic as I was led to believe, yet overall everything I read was accurate. (A good thing!)
Physically, as Keanex says, the build quality (or at least the appearance of build quality) is an improvement. These things look absolutely exquisite. Clamping force is harder on the 2000's vs the 900, but for me its still one of the most comfortable headphones I've had as my ears do not touch the cups.
Sound wise, the AD2000 is perfect if you ever said to yourself: I would love the general sweetness of the AD900 tone increased, with a more intimiate sound and a smaller and more cohesive soundstage (I always found the AD900 soundstage to be a little unrealistically stretched out), less treble in exchange for a smoother and very grain free high-end, fuller and tighter bass (you can hear every bass note, though sub-bass is still quite lean). The essential character still sounds similar to me in the AD2000 but the overall presentation is warmer and more involving, with less of the grainy dryness of the 900's. Clarity is better than my only other comparable phone, the Sony Z1000.
I can immediately see what people are saying about female vocals - there is a real sense of breath and air in them, so that they immediately stand out. The effect is quite stunning really.
My setup is: Music Streamer II+ -> O2 -> AD2000. I can tell you Keanex that the MSII+ and MSII are great, and one of the DACs recommended by the O2 maker besides the DACport. While I heard differences between the MSII+ and the MSII when I auditioned both, I would hesitate to spend much more on a DAC. The combination is very sweet and I think the O2 is an ideal candidate for low impedance cans like the AD900 / AD2000. Certainly better to me than the Schiit Asgard for my AD900 and Z1000.
I'm so happy to finally own these beauties :3
Edit: One note Keeanx just in case. On battery power, at 2.5x gain on the O2, the Music Streamer will cause voltage clipping because it exceeds line level spec a little. You must use the O2 on AC power or modify your O2 to get a 1x gain mode if in the rare case you want to use it as a desktop amp but still off battery power.
Edited by a_recording - 3/5/12 at 3:15pm