I first had the DT-990 @600 Ohms. However, I never liked them. They are nicely balanced and have good timbre, but they are far too bass bloated to be taken seriously, have awful sibilance issues (very similar to the HJE900) and just can't add the fun Ultrasone can, IMHO. So for me they were useless, except for the fact that I EQed them to a DT-880 so I could enjoy them as neutral and natural sounding headphones.
So for me, personally, DT-990 has no justification. The DT-880 is a much better design.
I guess it depends on which Ultrasone you have in mind. Their PRO900, for example, is one of the worst headphones to my ears with a brutally harsh, fatiguing sound. I found the bass impact unbearable on them - it felt like my head was going to explode from all the low end pressure. The mids and highs were cold, thin and peaky with a tendency toward nasty sibilance and a completely unnatural, synthetic timbre. I find DT990 Pro the better headphone in every way. Other Ultrasones are not so bad. I actually liked the PRO750 and DJ1 quite a bit, but their treble still sounded unnaturally harsh to me. I think DT990 does highs better. I don't hear any sibilance with my DT990 Pro, unless it's coming from the source. I do hear emphasis on the treble, but treble quality is fantastic with these cans IMO - the highs are smooth, ultra detailed and accurate. To be fair, I think DT880 600 Ohm has fantastic highs too, probably even better than DT990 Pro highs. It's mainly the mids, dynamics and soundstage that suffer on DT880 in comparison to DT990 IMO. I feel that midrange is simply more natural and revealing with DT990, even if it's a bit more recessed and the sound is more open and dynamic than that of DT880. The bass on DT880 600 Ohm is tighter than the bass on DT990 Pro (which is 250 Ohm), but I would never call DT990 bass bloated - it's still a very impressive low end and sounds more spacious, powerful and realistic than the tighter, but more restrained DT880 600 bass to these ears. Detail resolution in the lows, as well as in the highs, is fairly similar between the two models I think. Now, if you compare DT880 250 Ohm with DT990 Pro, the former falls behind in bass definition IMO with a noticeably looser sounding bottom end, not to mention that it is also more sterile and lacking impact compared DT990s. Overall, my view is pretty much the exact opposite of yours - I think DT990 is the superior version and is possibly what Beyer originally intended their headphones to sound like, while DT880 is an attempt to make a more neutral and analytical headphone that is better suited for professional use, but at the expense of some sound quality. For serious monitoring DT880 is probably superior, but for music listening and enjoyment, I would take DT990 any day over them.