I have the AD900s, but I've never tried the D2000s. Like some others here have said, the 900s have an incredible sound stage. Personally, I think the people who say the mids are recessed on these cans are pretty nuts - as I find the mids (especially upper mids) to be the highlight of these. If there is one place I would notch them down it would be the bass - but I do think they get a bit of a bad rap. The bass is definitely there. It is not particularly hard hitting, it will not make your head rumble from some of those deep drums, but it is in no way anemic.
Without a doubt I would say the ad900s are best suited for acoustics, vocals, and transients - they sound incredible. For Jazz and Classical, I would say these are good, but if you want cans that are exclusively for those genres, you could find better. Personally, I like my classical to have a lot of bass depth to it - I find the 900s lack in the lower mids (although the real low stuff actually is pretty nice) I suppose it depends what you listen to. Most stringed instruments/pianos/horns sound fantastic on these phones, but if your looking to listen to the 1812 overture and want to be smacked in the head when those drums/deeper horns hit - youll need something with more umph.
Edit: I forgot to mention, although I know most people here seem to frown upon it, these react fantastically to EQ. Without any changes I find them to be a bit bass anemic but the bass is really there if you warm it up some with the EQ. (I actually bring down the mids a few db and bring the lows up a few db to warm them up, giving a net gain of about 6-7 dbs in the lower end) The difference between no eq and with eq is pretty huge and its really only a plus.