First off I wanted to thank Zombie_X for this wonderful thread. It helped me in so many ways when deciding to purchase my new pair of headphones.
Thats what I ended up choosing for myself, very good headphone, but I highly recommend getting an decent headphone amp to properly drive them (if you are considering the 600 Ohm version).
I absolutely could not stand the DT990s, especially when compared side-by-side to the DT880. (And then to the HD650 for that matter)
Stand alone, without comparing it to anything, it sounds decent. But when I was shown "The Truth" by comparing it to the DT880, I was appalled at what I heard.
Beyers are great, don't get me wrong, but their sound signature is just too bright. It sounds like an "upside-down pyramid". Higher = treble, Lower = bass.
I compared both the DT990 and the DT880 before deciding to purchase my headphones. I was able to audition the 250 Ohm version of both. The DT880 is much more neutral and analytical compared to the DT990. I too found that the treble was very bright almost sibilant on the DT990, the treble was much less aggressive on the DT880. Yet what bothered me more was the difference in bass between the two headphones. The DT990s bass is truly exceptional, and when switching back in forth between the headphones all I could notice was the lack of presence in the bass for the DT880s. The bass on the DT880 is very good, but just doesn't have the oomph and punch that the bass on the DT990 has. So after my listening session I was already leaning towards the DT990, but the treble was worrying me. After reading many of the posts here, I ended up getting the 600 Ohm version of this headphone and paired it with a LD MKIII. This really helped tone down the "harshness" of the highs that I had experienced with the 250 Ohm version.
Overall if you really enjoy music with good bass, the DT990 is the beyer headphone for you. The 600 Ohm version manages to tame some of the harshness in the treble, while the bass will be tight, controlled, and will literally blow you away.
Yet I have to agree with Katun that the DT990 has the upside down pyramid or u-shape sound signature: Very present bass and treble, while the mids, although still very nicely detailed, sound slightly recessed.