Looking for a great tube headphone amp

The earmax pro should be on your short list. New for around $750 I think. It can drive both Grado's and Sennheisers.

Also, I've seen some used Stax for around $1000, that have a tube amp.

Depends what kind of music and what sound you like as to which is better. I've heard both...

I'm a relative newcomer to the world of serious headphone listening, but I'm using the Antique Sound Labs MG Head OTL with Senn 600s, and it sounds great. You definitely need to upgrade the stock tubes, tho! For $329 new, you really can't go wrong . . . .

I would also look at EarMax and Wheatfield Audio. Both units are tubed and in your price range. I am currently using the Wheatfield Audio headphone amp and have been quite pleased with it.

I have an MG Head DT that I'm pretty happy with. Sound improved alot with EI tubes. I have a pair of Sennheiser HD 600's. They are very accurate sounding, but somehow a little unexciting. I have a pair of Grado SR 325's that are a little more dynamic and exciting, but are more prone to ear burn and sometimes sound a little congested. I enjoy my Stax 303 classic set the most, though the sound may be dark to some ears. They are probably inaccurate compared to the Sennheisers. More fun to listen to though, and being primarily a speaker guy, I like them because they sound more like speakers. You might want to look into trying out a set of recent model stax lambda series (sr 303 or 404)headphones with a tube amp (SRM T1, SRM T1s, SRM 006t etc.) Older stax headphones tend to be a bit more unforgiving and hard sounding.

I would check out the Berning MicroZOTL and the EarMax (maybe the Wheatfield too...I haven't heard as much about that one).

You really need to listen to the Grado's and Sennheiser HD600 and make your own decision. They're both great headphones and both very different.That needs to be your decision, based on your own prefernce.