The AD2000 sounds a lot more Gradoish than the AD900, soundstage is smaller (but still better than what some people here claim it is), there's more speed, more aggression, bass is a lot more visceral than the AD900. Resolution and transparency is significantly better than the AD900. To me it's the RS1 with far superior transparency, far superior resolution, superior soundstage and a more tamed and less impressive highs. I think most of the hate for the AD2000 comes from the fit issues you will have for them on the start, they clamp down on your head tightly, they are uncomfortable and you might find the mids of the AD2000 honky. But those problems disappear after some stretching. Some people will complain too about the AD2000 being nasally, being too colored but that's the ATH sound signature. I think it's hard to argue that the AD900 is less colored than the AD2000. The AD2000 is unmatched as the king of mid-centric, all rounded headphones with a pleasant, fun, musical sound.
To me the AD2000 is certainly superior to the ESW9, especially if you are looking for something brighter, more airy and don't want sacrificing some bass quantity. Whether or not the AD2000 is worth the huge cost depends on how deep your pockets are, some will argue that it's not worth the jump over the AD700/AD900, which isn't completely wrong. But you can argue too that the RS1 is not worth the jump over the SR225/SR325, the ESW10 is not worth the jump over the ESW9, the D7000 is not worth the jump over the D2000 etc. IMHO the AD2000 is worth the jump over the AD700/AD900, unless you value soundstage over everything else in your headphone.