The DT-990s are currently my favorite headphone. I am very interested in getting Ultrasone. Since you have the K701s, We can compare there.
The DT-990s is an engaging headphone, but doesn't lack detail and has good clarity. The Sibilance factor isn't at all bad on the DT-990, unless you REALLY blast them. The sound stage is very impressive and enjoyable. The treble is bright and lively. Sometimes I feel it can be a bit too much, but the DT-990 spark emotions. The shimmer effect of highs is amazing.
It's more of the entire package rather then specific points. I feel vocals are more a part of the song, then made to stand out. The punchy trance bass smacks hard! It really is unrelenting and consistent. The headphone says, you will like the beat! The synthesized mids are warm and mobile. It truly becomes a concert hall like experience. Everything feels expansive and surrounding. There is an aura.
I will say this. After extended K701 listening periods, I put the DT-990 on and I say ... meh. Where's the sound stage and the amazing detail. The K701 is a GREAT headphone, when a song is produced well. The stereo effects from songs are brought to life and the vocals are amazing. It is truly an analytical phone, and becomes more enjoyable when you completely focus your attention. It focuses are that and it tends to avoid other areas.
HOWEVER, after listening to the DT-990s for a while, it seems like my ears adjust and then I love them again. The full, environment sound envelopes you. Then, after I listen to the DT-990 for a while, and put on the K701s, I say.. meh. Where's the warmth, where's the bass... etc etc.
What I love about this combination of headphones, is that they are different. I feel the need to listen to one or the other, and after listening to one, I rediscover the other.