Very nice write up, and great comparison, thanks for putting in the time. Although I agree with many of your remarks, I tend to disagree with some of your conclusions.
I agree that the DT990 does have a u-shape sound signature: emphasis on lows, recessed mids, emphasis on treble. Although the recessed mids may remove from a full bodied sound, I wouldn't characterize the DT990 as lacking body.
Here is my experience with the two headphones to present another opinion:
While auditioning headphones to determine which ones I should purchase, I was able to compare both the DT990 250 Ohm and DT880. When comparing them I immediately noticed the difference described by Katun: more bass and more treble. I must admit that the treble on the DT990 was even harsh on the ones I listened too, and I much preferred the treble of the DT880. Yet when switching back and forth, I couldn't help but favor the sound of the DT990. The DT880 has a more analytical sound than the DT990: less bass, more present mids, and less treble. All of these were very detailed and quite enjoyable. When listening to the DT990, the bass would always blow me away: it was powerful, had strong impact, was detailed and clean. I was surprised that an open ear headphone could deliver such good bass. Yet the harsh treble prevented me from appreciating everything I was hearing. So I left the store with quite a problem: I wanted the bass of the DT990s, with the smoother treble of the DT880s. After reading tons of post here I settled for the DT990 600 Ohm version. Paired with the LD MK III they sound amazing. The treble is no longer harsh and the bass is even tighter than before.
I think that these two headphones are really aimed at different audiences: the DT880 is a more analytical headphone with a slight emphasis on treble which make it a very revealing headphone; the DT990 is a more fun headphone with an emphasis on bass. In the end the DT990 just suited me better. Although its sound signature will not be everyones cup of tee, its the headphone that allowed me to appreciate and enjoy my music.