I say go for the MD's if you want good mids and more intimate sound than the D2000. The K550 are not what I would call intimate sounding. They are more on the brighter side with a bit of distances to the mids, airy soundstage (although at time I think the soundstage sounded slightly artificial). Bass is more focus on sub bass than mid bass, but at the same time I would not say their bass is anything to brag about or there strength ,then again their are people who say you have to get a good seal. I don't know what that whole good seal is about, but based on what I hear through them, I guess I'll put myself labeled as "not getting a good seal" even though I didn't see that I didn't have a good seal. with you coming from the D2000 and the HD202 to the K550, you will notice the lacking of bass. The bass isn't anemic to the point of feeling like there is no bass like the AD700, but still noticeable. I felt the K550 didn't do anything that wow me and the only thing that stands out with them which is the soundstage, which sounded off to me at times.
The MD's you can read everyone else post in the last few pages to get a good idea. The MD's are different enough to merit having both the MD and D2000.
I prefer the MD and the D2000 over the K550.
Edited by soundeffect - 5/10/13 at 9:26pm