While we're on the topic of the K501, I'd like to say something - they're definitely not everyone's cup of tea. My friend has a K501 hooked to a 220ohm resistance adapter to an Eddie Current Lunchbox tube amp (low price, great sound). I tried it out one day with a bunch of my stuff, but I wasn't too big a fan. Coming from a Grado SR60 (which are basically my favorite headphone sound; I'll be upgrading to a SR225 soon), the K501 sounded very "muted". The bass (and especially drums) sounded weak and anemic, and rock/techno wasn't very satisfying. I'm sure these are GREAT headphones for classical, because the soundstage was HUGE, the midrange was smooth, and the trebles were nice, but I really don't like them for gaming or anything. Another thing that I didn't like about it was it's reproduction of cymbals/hihats/drums, but I guess coming from a Grado headphone, that was probably to be expected.
Sometimes you have to give some ground to make some ground, so headphone selection really depends on what you like. The K501 may be the holy grail for some people, but for others like me, it wouldn't be used very often. I guess I'm just a grado junkie, through and through.