They're pretty interesting. They don't sound at all like the HD595's, the only relatively high end Sennheiser's I’ve heard in person.
They're very dynamic and realistic sounding. Bass extends deep but is surprisingly fast. For Sennheiser's I suspect they wouldn't satisfy a basshead as their midbass isn't bloated in any way; just fast, beautiful, textured bass.
Their treble isn’t recessed or particularly forward, it’s just where I feel it realistically should be. It’s very dynamic and actually gets shimmery and sparkly when present in the recording, something sennheiser's don't typically do.
The mids aren't overly smooth like other Sennheiser's or dry/analytical like in my K271MKII's. They're both very detailed and crisp and have just a touch of the sweetness typical to Sennheiser.
I believe the best term for these headphones is ACCURATE. Without comparing their sound staging/imaging to much higher end open end headphones from AKG/Sennheiser/Beyer or any of the electrostats, I can say these are the most accurate/realistic headphones I've ever heard, and they sound excellent on a 6G IPod classic 80 GB with ALAC files out of the headphone jack.
Edit: Though the HD560 may be the most accurate I've ever heard that doesn't exactly mean anything coming from me 
Perhaps the word "realistic" isn't up to par here, as their imaging with music isn't pinpoint accurate enough to convey a real sense of space as far as I can tell. I feel that my K271's beat them in this area entirely. In Soundstage size/imaging/pinpoint accuracy and depth the HD560 really seem to fall behind. On the other hand however, I do not have a source or amp capable of properly powering either headphone. The HD560 has a neutral sound that is both very full yet not warm. The AKG's have a thin but seemingly large, rich detailed sound very reminiscent of Etymotic.
The only reason I mentioned the iPod was becuase it can power the HD560. At an impedance of 300Ohm and a sensitivity of 94dB my laptop can't power them, even with the signal piggybacked onto the Fiio E5. My laptop can barely power the K271 with the E5. Aside from that I'm well aware of the fact that the iPod doesn't have great sound quality; I know its amp is pretty bad.
At any rate until I get a source capable of doing these headphones justice, I'll be keeping them both 
Edited by Sylverant - 5/15/11 at 10:41am