I had a recabled HD555 using silver based wire, a mod that opened up the top end and tightened the bass. I considered them to be mini-595's due to the mod. I then went to HD600's.
My impression is the HD600's win. They're simply better overall, and with a silver based cable and a forward source/amp they really aren't boring at all (the main complaint with Senn 580/600/650). For example, and EMU0404 with a custom silver breakout to a Gilmore Lite to a silver Senn cable. All of those components emphasize detail and treble and tighten the bass, so they match perfectly with the Senn to balance out the sound. I'd listen to rock all the time with the HD600's and they were great.
The thing I liked best about the 600's were the versatility. Senns are excellent for acoustic music, i.e. jazz, classical, vocal, etc. The sound was rich and very realistic. With processed music, i.e. electric guitar, techno, hip hop, etc. the sound was really detailed and clear. I eventually upped to the HD650's though since the 600's aren't very bassy cans, but other than the bass there's hardly any difference between the two.
--Illah