I can also compare DT990 to HD650 and HE-500 which I own and I have to say that while the drivers on the latter two may be technically superior (I am pretty sure HE-500 is, but HD650 may not be), DT990 actually sounds better damped and better executed to me than the HD650 and HE-500. I know I may get flamed for this, but I think HD650 is ever so slightly plasticy and congested sounding compared to DT990, while HE-500 sounds very slightly cupped in compared to the Beyers and I find both HD650 and HE-500 a bit slower on attack than is optimal, which takes a bit of life out of them compared to the snappier Beyers IMO. My sub par sources may be to blame for these findings, but I feel that these issues are not source related. Just look at the impulse responses of HD650 and HE-500 at Innerfidelity - they are pretty wild looking, which I believe is an indication of a non-optimal housing design or damping. Look at the DT880 impulse response by comparison - it is much cleaner. I know Innerfidelity doesn't have DT990 measurements, but I can almost guarantee that DT990 impulse response will still fair better than those of HE-500 and HD650. And compared to DT880... well... I think that DT990 sounds more optimally damped, while DT880 is damped a bit too much which causes it to sound dryer, more anemic, more sterile, more restricted than it should and doesn't quite do its driver capabilities justice IMO. I find that DT990 design makes better use of the driver, despite having more apparent (but less serious IMO) flaws. Again, it's all just my 2 cents and I am by no means an expert on these topics. I just go with my gut feelings on this based on listening and some data that I could find.
Interesting point correlating impulse response with cup damping. I wonder if this correlation holds with other headphones as well.