You'll get more responses on HeadFi
Check this out. I just ordered one and it's due in on 1/24.
You can also use it as a preamp, it has a 10 db gain.
I have some extra Mullard 6DJ8's and Amperex USN 7308's sitting around just waiting to jump in the Woo!
Grado just added and RA-1 HG designed for it HG=High Gain.
I realy like my Antique Sound Labs switchable transformer/OTL which at one setting or the other has enough gain for just about anything.
Buy a restored Fisher 500 receiver. The headphone output runs off the power amp outputs. Plus you get the tube phono stage and tuner.
Head-Fi is the place to go for advice. My advice is to look at Antique Sound Lab. I am using the HD-600 with their MG Head OTL Mark II. It makes the Senns sound sweet yet detailed. I can listen to them for hours. I wish now that I had spent a little more and gotten one of their similar pre-amps with headphone output. I would have made a small second room system by adding am amp and my new Von Schweikert VR-1s. Good luck!
Get a used Wheatfield HA-2 (they're now out of production and hard to find). There's one on Agon now for $600, or if you'd like to wait 2 weeks, I will likely be selling mine for $550 (once I get my speaker amps back, the headphones will unlikely see use for months). You will find no better Senn headamp for rock music, IMHO. Very fast attack, very full sound, and lots of dynamics.
Try the Xin Super Macro 3. I have used it with both the HD600s and the HD650s. I have the optional BB OPA627s installed on it and it is superb.
You might try a website called www.head-fi.com
It's a serious website for headphone enthusiasts - there are many writeups about headphone amps, headphones, cables, DACs, etc...