I'm certainly not going to dissent with you that the K501 sounds closer to the "real thing" than the K601 and K701, as I wasn't listening for that in this review. I was only trying to do a comparative assessment between the three here, nothing more. However, of all the headphones I've heard so far, the one that gets closest to the real thing for me has been the HD600. Not in terms of bass, but overall sound.
Maybe I did give the K501 the shaft in terms of my CD selection (I also added on a classical CD I forgot - thanks for a reminder), but I tried to give it as fair a chance as possible with the music that I found it did excel with. As I mentioned it did the best with orchestral and jazz, but I didn't mention it also sounded good with my Eva Cassidy, Azure Ray, and Alison Krauss CDs.
I think this may be a case of the variations in K501 units, because mine is clearly mids deficient. Eva Cassidy's Songbird is one of my reference points and I've been used to hearing it with the K701 and AD2000, which my K501 just doesn't compare to.
Not that I'm taking offense to this, but my CD selection isn't varied enough? I've got bluegrass, folk, rock, prog rock, trip-hop, breakbeat electronic, abstract electronic, film soundtracks, classical, acoustic piano, new age/orchestral, industrial, pseudo-metal, and ensemble acoustic. There's nothing there you don't listen to?
As I mentioned IRT Elephas' post, the specific K501 that I have may be to blame. I'm very sensitive to mid-range responses and I would've noticed. I really wanted to hear a solid mid-range from my K501 but couldn't detect one.
Clearly my CD selection isn't enough for you guys. Guess it's time to load up on some more diverse genres.