What headphone amp to buy-Sennheiser HD 600?

Md- Only you can decide about the ergonomics. The 6fg7 aka 6cg7 tube the stax uses is inexpensive. the SRM-T1 amp, IIRC is an older amp as are the Lambda pros and you should know that b4 you buy; also that service is provided on properly imported Stax cans by an outfit in Canada that provides good service, but occassionally they have to go back to Japan which as you might guess is expensive and slow. Gray market goods have to go back to Japan. If the amp works on 110 VAC w/o a transformer, it is most likely an authorized US import.
The 300b tubes came into the conversation via my recommended amp for the AKG-K1000s which are a very high impedence, unusual form factor hp that requires a speaker level signal of 4-5 watts or more.

Swampwalker:

As for the "gray" areas of the Stax marketplace, the pair I'm coveting are offered from the original owner, who purchased them from an authorized US dealer, so that won't be a problem. It's a great heads up in general, though. Price of repairs still COULD be a problem, and that's why what you said about the 404's is so interesting.

You said:

I can't remember why, but I think I preferred the Lambda Pros to the newer 404 series phone.

How big of a difference did you discern, if you can remember? The 404's appear to be the successor in the Stax line to the Lambda Pro Signature, although maybe that's wrong. In any case, the 404's can be had NEW for $499 at AudioCubes. So they could be an alternative if the price tag for repair of the Lambdas gets out of hand in the future, UNLESS you can remember something really bad about them......which is why I so selfishly ask you to jog your memory.
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Robm321 and Swampwalker:

It sounds like both of you, and many others, are impressed with the AKG 1000's. Do you think they'd work with the Odyssey amplifiers I have? What about the bass response of these?

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Marco:

You said (and Swampwalker too, but these are your words quoted):

I believe it uses 6CG7 (also 6fq7) - fairly common and not expensive. My preamp uses (only) two of those tubes and I like it very much.

Not to sound like TOO much of a cheapskate, but how much is "not expensive" from your perspective? Also, what about "tube rolling" with THESE tubes???
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EVERYBODY:

Thanks a lot for sharing your knowlege and experience and your patience in doing so! Every single post to this thread has been helpful in some way! (See optional footnote, if you want to...)

{{{{OPTIONAL footnote: This thread is fun--at least for me, maybe not for some of you (!!!)--but also it's demonstrating to me how little I know about tubes, and it's a strange feeling to confront my own ignorance on this topic: Marco, your kind words regarding my confessed "tubal retardation" are very much appreciated.}}}}

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Probably a typo on Swampwalker's part, but the tube is a 6CG7/6FQ7. It is not a 6FG7, which does exist, but I don't think is a drop in replacement for the former.

Inexpensive: read roughly $10-20/tube for good stuff. I've liked the GE's and JAN. There is one oddball tube I rolled that I cannot put my finger on right now that also was quite good. As I said, their a pretty common and not at all exotic tube. The differences in my amp, and those in your amp will not likely be the same. You should just get a few pairs and listen for yourself. I haven't found there to be as much of a difference in the NOS tubes of this type, than those in the very covetted, overused, much inflated, sometimes counterfitted 6922/ECC88 family. If your amp used those you'd be in for a roller coaster ride.

Marco

Marco:

Inexpensive: read roughly $10-20/tube for good stuff.

NOW yer talkin'!! Much, much better, for my budget anyway.

I think they would work with that amp. There are a couple of caveats:

One they need an amp with about 5+ watts - they connect to the taps like speaker cables do.

The second is as you mention the bass response. It goes very deep but is not as full as dynamic headphones. Most headphones have lots of coloration in the bass - the K1000 has the tightest and cleanest bass, but it doesn't get that flooded with bass feeling that you will get with Grado's and Senn's which you may or may not prefer.