I have the Grace 901 and have been very happy with it. It's compact, looks nice and sounds great. Combined with a zero sampling tubed DAC it's heaven. The Sugden is also supposed to be good, but I've not heard it. Blue Circle will have one this summer. You can also check out Positive Feedback Online for reviews of an Opera headphone amp and some Ray Samuels amps. I've tried the XP-7 and it's good too, but I prefer the Grace 901. A 901 just sold here for around $900 used, but it's hard to find them used.
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?