Nice, Don, maybe you should read what I actually wrote, instead of commenting on what I didn't?
*) There is no doubt cryo treatment - along with other temperature treatments of many metals and alloys - can change their mechanical properties, and thus for instance increase wear resistance, bulk resistivity and other physical properties. This is a well established technique by now.
*)
However there is no mechanical wear on a tube in service, unless you hit it with a hammer. A tube wears out due to - among other things - chemical exhaustion of the cathode, contamination of the cathode due to migration of impurities from the cathode sleeve (usually made from a very pure nickle alloy to try and prevent this from happening), and slow outgassing from the metals and the glass making up the tube. Sometimes you have slow leaks at the seals as well. It is not clear to me how improving the mechanical qualities of the bulk metal making up the tube structure can prevent these processes from happening, or slow them.
*) Tubes have been space rated since the late fifties. I would be extremely surprised if part of that certification of different tube types didn't incorporate some form of extended storage at cryogenic temperatures a bit above the absolute zero. The intention being to see what would happen when a satellite transmitter is powered up again after a prolonged break from service. Age tests would be the logical extension to see how the abuse might have influenced the useful service life of space rated tubes. I have never heard of any such effect being observed. Please give any references if you know of any such studies which did see it, involving the usual 5 to 10 thousand tubes.
*) What you describe as a cryo treatment is really a 'dip' by modern standards, considering the testing modern, high power tube equipment is subject to as a matter of course when being man/life rated. The (N)MRI scanner at your local hospital is likely to still contain quite a few serious power triodes, probably made by
these guys. Being able to, say, double the time between scheduled tube replacements in those settings would be a big deal if something as simple as cryo treatment works. You could really make a killing in that industry with an effect like this.
*) I would be highly surprised if an effect like you describe, an unknown mechanism extending the lifetime of low power vacuum devices via cryo treatment, exists, and at the same time hadn't accidentally been discovered by half the physics departments of every reasonable university in the world. Those places use liquid Nitrogen by the truckload, literally, and not (only...) to cool their beer.
*) I didn't say anything about how such a treatment might influence the sound of a tube. But since you did, please allow me to comment. Did it occur to you that we are actually not listening to the metal of the tube, but to the structure of the vacuum in between it?
*) It is for you to decide whether you waste your money on any service you buy, not something I can nor will comment on. Please leave the ad hominem attacks at the door if you wish for us to continue this conversation.
Frank.