Well, I took the plunge and bought the HD380 Pros ($238 including tax and extended warranty.)
So far, I am very impressed. Far from being "one step up" from the HD280s, they are in an entirely different league. Indeed, if I had bought HD650s and this is what they sounded like, I would have felt like I got my money's worth!
I really never expected any closed phones under $500 to outperform my DT150s, but these do. Unlike the 150s, these are also supremely comfortable.
The first thing that I noticed was soundstage. As in freakin' immense soundstage! There is absolutely nothing to suggest that these are closed cans, because they sound like transparent screens that the music passes through. I had K701s for a while, and they had nothing on these in terms of either openness or soundstage.
Next was detail. Tons of detail. I picked up on subleties I have not noticed in very familiar recordings, always a sign of great cans. Again, the K701s have nothing on these.
Overall frequency response seems a bit tipped up, with the treble being quite prominent. Because the treble is so detailed and smooth, however, this does not make them sound harsh or fatiguing (so far.)
At first, the bass seemed weaker than DT150. After a bit, however, I realized that it was just not calling attention to itself. When the recording contains deep, powerful bass, the 380s rise to the challenge.
The only negative I can find is that they do have the same issue as the 280s with revealing the noise floor of my amp. Unlike the 280s, however, these are good enough to justify buying a dedicated amp at some point.
In summary, these are both everything I had hoped for and much more than I expected for the price: All the SQ that the very highest-end open Sennheisers are known for in closed phones with good isolation and superb comfort!
At this point, my plan is to let my friend at work choose which pair of Beyers he wants to keep (DT770 or DT150), and sell the others. That is a pretty amazing statement coming from me, because I was a huge fan of the Beyers until I heard the HD380s!