I have to admit, I'm going on memory also, which is long enough ago that it may be failing me as well. Also, maybe because the DT880 didn't exist at the time, compared to the DT770 it was bright and not bass heavy. I do agree that I found the DT880 (once it came out) straightforward compared to the bass-heavy DT770 and the (to me) bright DT990. I guess I think of it like this:
DT770 \
DT880 -
DT990 /
(Bit of an exaggeration, but it's the best I can do with ASCII. Imagine me standing in front of you with my hand in the air with a slight slant representing bass-heaviness for the DT770, etc. No, no, I'm not right in front of you, crowding your personal space, I'm standing next to you holding it out in front of both of us.)
Also, I have an old (pre-DT880) DT990, so maybe they were different. I did have an HD600 at the time, and remember it having more bass than the DT990, but I don't remember what I had at the time.