Oh my, the M220 sound lovely. Much warmer sounding than the K271s, and really what I was looking for. All my other headphones are a little on the forward or sterile side, so these have found a nice place in my collection.
EDIT: So I think I like these more than the K612s. The K612s have a larger soundstage, but the imaging is missing a lot between the channels. Either everything is a tiny dot in the dead center, or it's far to the left and right. With the M220s, the soundstage is still big, but there's a smoother transition from middle to sides. It's much more natural sounding.
The mids are also better to my ears. Less forward on the upper end, and much more lush on the lower end. This really meshes well with the music I listen to, since most of the singers I listen to have grainy and/or harsh voices.
The bass doesn't extend as low, but it feels like it has a little more impact. Accentuation creeps up into the upper bass regions a little more, too? I have a terrible ear for bass, no doubt thanks to the XB700s, but I think that's what I hear. Gives it a warmer sound overall.
The detachable cable is also a HUGE plus for me. I'm always afraid I'm going to run over the K612s long-ass cable with my chair and wreck it. With these if I do I can just replace it. :3