If you like the tube sound, I recommend checking out the ASL OTL32 headamp/preamp or the ASL Twinhead headamp/pre or the RKV. If you want solid state, I think you'd be hard pressed to find anything better than a PPA amp, regardless of price. Several head-fi memebers said they heard a PPA amp with the HD650 at a meet and it blew away the Headroom Blockhead. So you could go with a PPA and save quite a bit of money. There are tons of amps out there, so be careful and don't lose your head over this. BTW I have the HD580 with MG Head OTL and I think it is a wonderful combo.
Best headphone amp for new Senn 650's
I just ordered a pair of the 650's with an upgraded cable from Drew at Moon Audio. I have been over at head-fi.org and Headroom Corp. and looked through these forums too and have come up with several possibilities for a new headphone amp. My question is, is there something that would considerably improve upon the Antique Sound Labs MG Head (older version with transformer, and Telefunkens) that I currently use with my Senn 600's? I am tempted by the Headroom new Max, the Grace 901, the Sugden Headmaster, and various other esoteric things I read about at head-fi. There are a lot of headphone listeners here, too. Any thoughts on whether I could substantially improve on the MG Head? Maybe other tubes than the Telefunkens I now use?