Question about Stax Lambda Pro Headphones

I have an old pair of these headphones, 1980s vintage. They are driven by an "ear speaker adaptor" which connects to the speaker terminals of a power amplifier. Question #1: Is it safe to use these with a tube amplifier ? In other words, when the selector is switched to phones, does it present a safe load for the amplifier ?
Question #2: Is there a tube headphone amplifier available that will drive these headphones ? Preferably one that can be driven from pre-amp outputs ?
Q1) Good question. I couldn't find input impedance specs on the 1980s Stax adapters, which would allow a definitive answer. FWIW, though, my guess is that there would be no problem. Not sure I'd bet the health of my amp on it, though. I did find the input impedance of the ca. 1970 Stax SRD-5 described here as follows:
Not more than 30 Ohms anywhere in the audio band. 5 Ohms DC.
If the spec on your adapter is similar, which I would guess it is, there would be no problem. A problem might arise if the input impedance is significantly higher than that, in which case potentially damaging or reliability-degrading "inductive kickback" could occur in the output transformer of the amp (assuming the amp has an output transformer).

Q2) Sure. Stax itself makes several models today, as well as Woo Audio and possibly one or two others. They are not inexpensive, however.

BTW, I also have a pair of Stax Lambda Pro headphones, which I purchased new in the 1980s. I recently acquired a Stax SRM-T1 tube headphone amplifier, ca. 1990, at a small fraction of the price of the current Stax models. It replaced the solid state Stax SRM-1/Mk-2 amplifier and Stax ED-1 Diffuse Field Equalizer combo I had been using for the past 25 years or so. I'm VERY pleased with the improvement. SRM-T1's don't seem to appear for sale very often, but if you see one in good condition grab it!

I've connected these Stax models, btw, to the tape outputs of my preamp, not to the main preamp outputs (which would just introduce unnecessary circuitry and an unnecessary volume control into the signal path).

-- Al
Al's comments are on the money; as usual. I too am a long time user of both the Lambda Pros and Lambda Signatures. Before replacing the SRM-1/MK2 with the SRM-T1 as Al did, I used a Dynaco ST70 to drive the Pro's via the Stax Energizer (adapter). The results were excellent and I preferred the sound over the sound with the SRM-1 which, while very good, sounded clinical and too tipped up in the highs for my tastes. I can't imagine any problems with any decent tube amp. The SRM-T1 is excellent with the Pros and is the way to go; they show up used on a fairly regular basis. Mine is actually the SRM-T1S; essentially the same but with balanced inputs as well as RCA and slightly different cosmetics. Good luck.

BTW, Al, what are your feelings about the ED-1? I like mine very much, but have always been mystified about the claims of improvements in the spatial qualities of the sound. I don't really hear that, but do hear significant improvements in timbre and tonality and a little in dimensionality.
Frogman, thanks for the input about using the ST-70 with a Stax Energizer, which of course adds confidence that Mabonn would be ok using his with a tube amp.

Regarding the ED-1, my perceptions have been exactly as you describe. Its most notable effect IME has been to mitigate the tendency of the Lambda Pro/SRM-1Mk-2 combination to sound thin and lacking in harmonic richness, most notably in the upper midrange and lower treble, together with excessive brightness further up in the treble region.

It accomplishes that, however, at the expense of some sacrifice in transparency and definition. How that tradeoff nets out has seemed to me to be recording dependent. And more often than not I had found myself switching the ED-1's equalization off, especially with recordings that are well engineered.

In any event, that mitigation no longer seems necessary now that I'm using the SRM-T1, and so I've removed the ED-1 from my system.

A further point that seems likely to be relevant to Mabonn's situation: Ever since I purchased the Lambda Pro's ca. 1986, and continuing to the present day, I've consistently noticed that if they are not used regularly, and especially if they are not used at all for a period of say a few weeks, they need to be given a good workout to again sound their best. By "good workout" I mean playing them at very high volume (louder than would be safe if they were on your head) for two or three hours or so. Otherwise the negative tendencies I mentioned in the second paragraph of this post tend to become considerably more pronounced.

Best regards,
-- Al
Thanks for your responses. This is very helpful. BTW, my tube amp is an ARC VT100 Mk II, which does have output transformers. Also, the adaptor I have is the SRD7.
I am interested in the possibility of finding a good used tube headphone amp that is compatible with these phones. I guess that means that it compatible with the cable that connects the phones to the amp, and supplies the appropriate voltages to charge the electrostatic elements. If it has a volume control, all the better, so that I can drive it from the record outputs of the preamp. I am assuming that a good tube headphone amp would be a sonic upgrade from the adaptor I am using now (SRD7).
I am sure that Al can do a much better job of explaining this, but my understanding is that it is not simply an issue of connectivity (cable/plug) compatibility. The Stax tube headphone amps are designed specifically to power their headphones; I don't think that, for electrical reasons, any other tube headphone amp would work. I have seen and read about one or two one-of-a-kind tube amps made by hobbyists for use with Stax cans, but am not aware of any made commercially other than the Stax amps. Additionally, I would not assume that a good tube headphone amp (even the Stax; as good as it is) is a sonic improvement over the SRD7 with a good tube power amp like your AR. IMO, if you are primarily a headphones listener and if you can live with the somewhat subtle sonic degradation to the sound from your main speakers as a result of using the SRD7 (I couldn't), then it seems to me you already have the best, or very close to the best, possible sound from your Stax cans.
Yes, as far as I know the only headphone amps suitable for use with the Stax "Pro" phones, which would provide the right bias voltage (580 volts), the right signal characteristics, etc., are those specifically designed for them. As I mentioned, though, Woo Audio and one or two others manufacture such amps, as well as Stax.

Here are links to the currently produced Stax models.

Links to the Woo models are near the bottom of this page.

Here are links to a couple of others.

I have no first hand knowledge of how using the SRD7 vs. one of these amps may compare sonically. I can say, however, that I doubt that anyone's sonic perceptions can be counted on to be more reliable than Frogman's.

Also, member Larryi, whose perceptions are also top notch, provided comments about some of these amps here.

-- Al
Thanks for those links, Al. Man, I had no idea. I have to admit that those amps look very interesting. For now, I am content with my Stax amp; but.....
Back in the 1980s, shortly after the Lambda Pro came out, I listened to both and thought the Pro sounded too bright (at least to my considerably-younger self), so I picked a pair of the considerably-cheaper regular Lambda. I have not heard any other Stax phones since then. 

I still have them and am still happy with them, and have used them all these years via the SRD-7/mk2 hooked up to a variety of power amps. Back in the day, I listened to headphones a lot more than I do now.

Now it’s an occasional thing. With all headphones I get a tiny soundstage right behind my forehead, but other than that, I have always been happy with them.

This is probably an extinct product, but I have a 25-foot Stax extension cable. The Pro has a five-pin plug, the regular Lambda has a six pin plug. This cable has a six-pin female end, so you can attach it to any pair of regular or Pro Lambda earspeakers, and a spring-loaded 6th pin on the male end that will retract if you plug into a five pin socket. 

The danger is is that the retracting pin will let you plug your non-pro phones into a pro-voltage socket when using this cable. 

The SRD-7/mk2 has one of each socket on the front. I put a sticker over the five pin socket just to be sure nobody ever plugged my phones into the wrong one and fried them.