Best $300 or less Tube Headphone Amp

Have you checked out www.head-fi.org? They sponsor a set of forums dedicated to headphone users, including a forum on headphone amplifiers. With a little searching you'll probably be able to get some specific recommendations for your particular headphones. Good luck.

ASL MGhead. I use it with my senn hd600, and it has good synergy.

Also, the high gain grado RA-1 actually has a little more detail but you lose some of the tube qualities.

ASL's MG Head DT OTL, now in MK III version. (MK II and III have separate power supply.) You should be able to find a used MK III, and certainly a MK II, for under $300.

I have the Channel Islands with my 600's and it is an incredible pairing. Just a hair above your budget, but worth it IMO

I like the Musical Fidelity v3