when doing some real listening, i can't imagine being under... say 60 or 70db. else the soundstage collapses and all music with a high range of dynamics would result in not hearing the most quiet sounds. (i love normalizers for movies but i can't resolve myself to massacre a fine piece of music with those tricks)
on the other hand when i listen to music at home while doing something else, i would go lower and lower in volume over time and end up, i guess, around 40db (that would be the fans of my computer level of sound). with such low volume, everything is a mess from soundstage to bass. i gave up on soundstage as i'm not really listening, but i tend to go for bassy headphones on those occasions to keep the "feel of music". that way when the bass begins to fail, there is still enough to be neutral. given that those small bass left have the ****tiest damping you can dream of.
if you can't stand badly controlled bass, i guess something closer to a monitor would be best.
ps:got a hd650 (again) recently and i must agree with the others, it is surprisingly nice for quiet listening. just don't expect to hear the hd650 at its best, but details are ok and the smooth signature goes well with the idea of relaxed low volume listening.
I often listen at low volumes. My room is pretty quiet so there isn't a heck of a lot of ambient noise. 45 - 50 dBA is quite sufficient in such situations. I can go even lower, around 40 dBA. Below that, and music sounds more like a whisper and is not especially rewarding.