I've not heard too many much better when it comes to pure sound quality. Although I think they have some major drawbacks-- namely lack of bass and treble extension. The silver damping version I would consider reference...
I agree with rhythmdevils, I had a short listen to them and they sounded a bit tinny and thin. Typical sound of underdamped plasticy enclosed backs. Their clarity was quite good but they had timbre and damping issues to me...
If you feel like the D2000 can have bothering treble, stay away from the DT990s. Also I wouldn't consider DT990 a good enough of an upgrade over the D2000 since they share many similar qualities.
I have the HE-400 yes. With the velour pads, the fit's a bit more finicky than the Denons, but your ears will never once get sweaty. Not to mention the sound will most likely be an upgrade in all areas. DT990's bass...