What headphone amp to buy-Sennheiser HD 600?

Quick update:
The Stax headset has returned from Yamasinc, and has been rebuilt to original Lambda Pro Signature status. They look beautiful. Price for newer model 404 would have been almost identical, but I opted for the restoration based on what Swampwalker had said about having heard both units, what they had sounded like before I broke them with the vacuum cleaner (yes indeed, quite an idiotic thing to have done, yes indeed....), and based on a professional review I found somewhere on the 404's that said that there was a peculiar artifact in the bass response that was absent in earlier versions. Two weeks to break in, then I'll have "NOS" Stax.

Main reason for posting: Yamasinc appears to have done just an excellent job, and that's worth knowing for other Stax owners who may be considering upgrades and/or maintenance with that company, which is the authorized U.S. distributor.

Although the headphones look beautiful, I was mistaken that they had been refitted with the original diaphragms that were used with the Lambda Pro Signature earspeakers. That diaphragm was discontinued several years ago and is no longer available. Therefore the repaired unit is esentially the 404, with its thinner diaphragms, according to the technician I spoke to at Yamasinc. They sound very good, but the original owner heard them on Sunday and said they didn't sound as good to him as when they had their original old Lambda Pro Signature diaphragms. Oh well, they still sound very good. But Swampwalker, you were right it would seem.

Md- Please forward the final sentence of your post to my wife ;~) Glad that Yamasinc was able to hook you up.

Now that the headphone system is up and running (I still haven't worked up the courage to change the tubes on the SRM-T1), it sounds excellent. It could still sound better, in my opinion. So.........what do you guys think of the ED-1 "diffuse field" equalizer made by Stax? Would this fill out the bass without introducing too much distortion, or is it more of a device to create an enhanced "soundstage" for the headphones? Is it worth pursuing one of these units which originally retailed for around $600? If so, what's a good price, and where can I steal one?

Epilogue:

Having destroyed the diaphragms on the original Lambda Signatures, then having them rebuilt at Yamasinc to essentially 404s with the new diaphragms, I wanted to see how the ORIGINAL version of the Lambda Signatures would compare. I found a pair in great condition and bought them. Because there are TWO pro headphone jacks on the SRM T1, I was able to compare the two sets (newer 404s versus older Lambda Signatures). The differences were definite and obvious, but not earth shattering. Through the SRM T1, which is tuned to the original Lambda Signatures (according to Yamasinc), the 404 equivalents were significantly more efficient, with higher output at any given volume setting. The 404 equivalents were significantly more detailed and crisp, and probably extend a little deeper in the bass. In comparison, the Lambda Pro Signature originals were sweeter and probably more "musical." When listened to at the same volume, the two were quite close. I preferred the sound of the 404 equivalents, but an audiophile friend of mine felt just the opposite, and we each listened to both of them at the same sitting. One of us could listen to one pair while the other one listened to the other pair on the same track.

Alas, after all that, I ended up selling the whole Stax headphone system and also plan to sell the Sennheiser HD 600s. None of them sound good enough compared to the Intuitive Design Summits; it's as simple as that. I'm too spoiled by the sound of the mains. The only thing I can think of left to try is the Omega 2s. I'd certainly use the tube 007t if I got them, or else something from Singlepower, but we're talking about big bucks here, so it'll have to wait, if it ever even happens at all. Plus, there are no guarantees I'd even be satisfied with THAT. Kind of frustrating, but I'd rather go without any headphones at all than listen to something that doesn't satisfy my sonic demands.

Sincere thanks to all who posted to this thread trying to help with the selection. I now can say that I've ventured into tube based amplification.