I agree on the DT250-250. They have a certain soft focus quality to them. Not terrible, but not quite up to reference grade standards. They are a casual listening phone with a very natural sound (to me, anyway). The balance is quite similar to the Ety ER4S, I think, but with beefier bass. I would love to have a pair of canal phones that sound just like the DT250.
A trick to help with portability is to take the cord off (I never install that lock screw). Without the cord they can be collapsed to a very small package. I also just hang them on the shoulder strap of my junk/laptop bag (can't really call it a briefcase anymore).
The DT770 pro had a distinct 3 band response, with a huge overboost in the bass. What bothered me the most was the boost (tuned resonance, maybe?) went clear up to 150 hz. boosted mid and upper bass gives the music a thick sound with the "boomy cheap speaker" aura that I seem to have developed an aversion to. This was also an issue with the new UE phones that I tried. Just not what I am after.
Since I seem to be rambling on about bass tuning, the Ultrasones seem to have the boost in a nice location. The penalty is that they have a matching jump in the treble range. The Ultras isolate better than the DT250.
Apar, you expressed the default head-fi solution: "keep them both"
Good luck with the new cans.
gerG