The K550s have more bass than the K701/2's.
I have the DT770 80 ohm and really like the bass. The k550 seems to have a pretty deep response however not quite as sub bass heavy as the 770s.
I crave sub bass and midbass seems to mud things up too much for my tastes. The 770's dont fatigue me either as many headphone makes will rolloff that freq (5khz) to reduce fatigue. The 770s can sound dark, so beware if that bothers you. I would avoid the sony XB series at all costs, its really terrible. I've been considering the LCD2's lately but I worry that the soundstage, open design and comfort factor may disappoint.
Freq Response here
Beyer DT770 (Note low distortion, decent sub bass, also note larger dip at 5khz and spike at 9ish khz. The 80ohms will have even more sub bass than the identified 600ohm pair)
AKG K550 (Note higher distortion, decent sub bass but higher distortion and smaller peaks)
AKG K701 (Note rolled off bass and high distortion, highly overrated IMO)
Edited by Docks - 12/2/12 at 5:26pm