I drive my Senn HD650's with a SI T-Amp via an ASL UHC with spectacular results. Prior to installing the T-Amp, I had planned to purchase a Singlepower MPX. I found a local headphone nut who happened to have a Supra on hand and invited me over to give it a listen. It's a great amp, no question. However, the following week, I picked up the T-Amp on a whim, and well, it comes frighteningly close to the Supra.
While I don't love the T-Amp with my speakers (4ohm, 96db), it really seems to shine as a headphone amp. The T-Amp drives the 650's easily, and they have great synergy. I highly recommend this combo. Of course, you need a way to connect the headphones to the T-Amp, this is where the ASL UHC (universal headphone connector) enters into the picture. It's passive, inexpensive and effective. However, I had much less success with the ASL when using my Korneff 45. The ASL seems to not infringe on the music at all with the T-Amp.
I'm quite pleased with this setup. Unfortunately, I don't think I've played it for more than ten hours tops. I have so much invested in my big rig, I feel guilty listening to the headphones. Plus, I just don't find much time to listen to cd's, and I have my vinyl set up connected to the Korneff.
Anyway, if you plan to spend a good amount of your time listening to headphones, don't settle, a great system can be had inexpensively. When I was shopping for headphones, I was able to audition side by side Senn 580, 600,650 and Grado 225, and 325's. The sales folks were really busy that day, and I think they just forgot about me, as I was upstairs in an alcove. I was able to listen uninterupted for about 3 hours. The 650's seemed to be the most dynamic, detailed, and open, with great image and outstanding (for headphones) soundstaging.
Good luck and have fun