What headphone amp to buy-Sennheiser HD 600?

If you have those high demands get the AKG K1000. I own Grado RS-1 and Senn hd600 (heard the 650) - I heard the K1000 and end of story it was in another league altogether while creating a soundstage outside of the head (by a little bit). If you can hear them do so before centering a system around the Senns. I wished someone would have told me about the AKG's before I went down that road.

Robm321, and everybody else who has contributed to this thread so far:

Thank you all very much. Headphones are a whole new realm for me.

I highly recommend the Singlepoweraudio.com tube amps for the Senn. headphones. I have owned or heard the PPX3, MPX3, SDS, and the SDS-XLR balanced amps. The base models are the best headphone amps I've used, and the owner will custom build any amp to your phones or needs.

Sugden Headmaster, it fits all of your requirements I guess: solid state with tube-like sound, built with HD600 in mind, remote, excellent line stage as well, Stereophile Class A headphone amp (for what it is worth), about $700 used. Only issue is to find one second-hand. I had been looking for one for over a year and found one on Agon...I was read number 8 on the Ad and bought it right away! I sold it since (no time for headphones) to someone who had been looking for one for 18 months...no depreciation, I can tell you this!

http://www.onhifi.com/product/sugden_bijou.htm

Quick Update, and request for additional opinions:

I just listened to a Stax Lambda Signature Pro System using the Stax SRM T1 dedicated vacuum tube amplifier. The signal is routed to the headphone amp thru the same player, dac, and preamp as the Intuitive Design Summit Loudspeakers and the Rel Storm 3 sub bass unit are.

The phones don't express the bass as well as the speakers driven by the Odyssey Extreme monoblocks and supplemented with the Rel Storm 3, but my guess is that I shouldn't expect that. They aren't BAD in the bass and it's certainly tolerable. There appears (on initial listening) to be a touch more detail, but not nearly as much more as I had expected: the difference is not that impressive. Furthermore, the high end is not as sweet as the Summits', but it isn't TOO harsh, and my guess is that the vacuum tube output stage is helping in this regard. Midrange proficiency seems overall similar between the Stax and the Summits; there MAY (or may not, I can't be sure yet) SLIGHTLY better midrange detail with the Stax, but timbre and tonality MAY (or may not, again) be SLIGHTY better on the Summits.

I'm sort of (but not really, I guess) surprised that the Summits hold up so well against these esoteric electrostatic headphones, and in fact, the Summits plus the Rel are CLEARLY the preferred listening mode, especially when soundstaging and bass reproduction are factored in. However, in the other areas mentioned, these seem to be at least close to the Summits, so they should be listenable in the long term (at least during the short interval between when my wife makes me turn off the Summits after 9:30 pm on school nights and when I go to bed).

All of the above having been said, here's the decision to be made: whether or not to purchase the Stax Lambda Signature Pro Earspeakers plus the SRM T1 vacuum tube headphone amp for the price of $850, all inclusive. I'm told that this 10+ year old system (which is in very good condition, by the way), originally retailed for about $2500.

So, what do you guys think? In the absence of strong advice against it, I'm about to pull the trigger.