Looking for a great tube headphone amp

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.

I've owned an MF X-CanV2, Wheatfield HA-1, and ASL MG Head OTL. My preference and current amp is the MG Head. I like a tubey laid back amp with headphones so I use the transformer output rather than the OTL. All three are very good headphone amps. The Wheatfield can also serve as a single input preamp (which is how I got into the whole tube-based system). As described by the folks at the HeadRoom, the Wheatfield is less tubey and more dynamic. That same description has been used when talking about the Berning microZOTL (which can be used as a 1w amp). For a little over 300, the MG Head is hard to beat though. -- You can find interesting discussions of these and other headphone amps at www.head-fi.org. Good luck.

Don't rule out the MELOS SHA-1...

I'll cast my vote for the ASL MG Head amp driving the Sennheiser HD600. I don't find it excessively tubey, very good bass, excellent dynamics and revealing without being brittle, an excellent combination!

I second the Melos SHA1 or its relatives. Sounds very, very good and reasonable from a tube rolling standpoint. You get a pretty nice pre-amp in the bargain, too. I am a dedicated Stax man, but appreciate the Melos, also.