For me, it's not just the music but the headphone and amp too. With some headphones I just find it too hard to want to review them and sit there and over-analyze them. With some headphones I love them so much and they're so engaging that I could care less about their detail, lack of detail or how clear the sound is. Often when I listen to a headphone that I'm reviewing I'll just sit there and forget I'm supposed to be doing a review! I just get too engrossed into the music I'm listening to.
The K501 is like this for me. It's not technically as good as the K702, but I could care less. Less detailed, less clear (maybe) and has less treble, but with such good mids, who the heck cares?
It kind of has a relaxed sound, but still quite engaging. I could probably listen to them for 5 hours or even fall asleep with them on my head. I love a headphone that seems to get everything right based on your own preferences. I don't know why but it seems this is rare. So far out of the dozens I've tried it's only been the HD-598, DJ100 and now the K501. K501 is closer to my perfect headphone though. Only took me about forever to find it. DJ100 isn't a good headphone to relax with since it's a tad too aggressive at times, but depends on the music. HD-598 and K501 are far better for relaxing. Another one that would be great is the HD-600 I think.
BTW you know a headphone is good for this when it makes you stop buying other headphones or wonder why you always bothered with getting so many in the first place.