I've got a pair of the 81s, too. A little bass heavy, but really nice overall, especially given the price. Definitely great for rock music though - the bass, despite being a bit too much, is actually quite detailed so it doesn't come across as boomy. I actually haven't listened to my HD580s since buying them.
I picked up this tip on headfi for an easy and effective mod - the felt discs that cover the drivers can be removed (there is another piece of fabric beneath that). It brings out the treble and midrange a bit more and helps even out the bass some. Simple and easily reversible!
As for your choice, Dan, I think you made the right one given your music preferences. I have the HD580 and it is quite power hungry, and I can definitely tell the difference in SQ between my heaphone amp (Gilmore Dynamic v2) and the headphone jack on my receiver. It also has a pretty laid back presentation (where some headphones/speakers might be described as giving you a front row or on-the-stage presentation, the 580s sound like you're a few rows back in the audience), so it isn't the best choice if you want to rock out. That said, they have a huge and very precise soundstage, are very well balanced, are extremely detailed without being harsh they're very comfortable, so I can wear them for hours on end without any fatigue. But if you listen to rock and want a phone with a similar sound to the M22, then the 580 ain't it. I think mine makes a good counterpoint to the Axiom sound, though.
