Mike,
I had a very interesting conversation with Dennis Boyle the other night that started out dealing with cryogenics. Possibly the straightest, cleanest explanation about why a NOS tube should sound better.
As best I remember,
Metal has a built in memory and over time tends to restructure itself back to its original form.
When a tube is made the solder joints and other metal stressing operations will leave the metal changed.
If a tube lays on a shelf for 50 years the memory will erase the stress of manufacture.
So a NOS tube should sound better.

The flip side to that is that since tubes are hot, that over a period of usage annealing will take place and once again change the structure.

I guess the answer is to buy up a large stock of your favorite NOS and keep track of the hours used and rotate them out before they change too much. You can only wait 50 years so many times.

Don't get me wrong...there are many NOS tubes which I do like...and I am by no means against NOS tubes...or new ones for that matter.

But, these tubes are built with certain specifications in mind...and measurements(and if appropriate) and listening tests are done. If a tube changes it's "sound" thru sitting on a shelf or thru a various cryo method...there is NO telling if the tube will have a better or worse sound for audio. The metal will go back to it's natural state...but the tube was not designed for the metals natural state. Hmmm...I get the feeling that is making no sense what so ever.

To me...I can see the appeal of cryo'ing tubes. It almost seems to have a purity about it. restoring things the way they were at one time. But not one piece of audio was built with the intention of having it cryo'd...but many audiophiles believe if they get something cryo'd it will have better sound. I just don't see this as being so. If it changes the metal...couldn't have a better chance of having worse sound? If this specific thing was designed to perform well in it's assembled state...then any alteration from that would have a slightly better chance of being worse than good.

I will say that I have not tried any piece of audio equipment that has been cryo'd. I could see how it CAN make things different...and sometimes better...but what about the other times?

ok...ok...I'll shut up now

It is definatly a two sided argument and while not all change is good not all is bad. It depends on what you want and why you are doing.
This is hearsay, or heresay, but here goes. Rifle freaks will cryo the very best match grade barrels to shoot tighter patterns. Some race car engineers will cryo an engine to get better performance and certain types of guitar strings are supposed to sound the very best if cryo treated. Acoustical I'm thinking and I have heard whispers in the news groups about one HiFi maker that cryo treats their preamps.
If sperm can survive it , well..............

Thanks so MUCH for sharing your conversation with Dennis with us. Personally, this is exactly the type of exchanges I enjoy. Back on topic. I've NOT heard of this "compound material setting process" before, then again I'm pretty ignorant about such things. I rather deal with the effects and not so much the process(s). I'll have to bounce that off of those more knowlegdeable than myself - sounds very interesting! As far as NOS tubes, I must admit that I generally persue the best / oldest tubes possible. One does not always promise the other but usually the older tube will be perferred. That is my experience with rolling tubes the last five years. I do not know of one current production tube that is superior to NOS. Well, I hear the new Emissions Labs 45 tube is reported by many to be superior but at $350 a pair I'm not so privileged.

CRYO treatment: Again, I'm familiar with the process but not in great detail. I have experience with cryo treatment on two devices; Mullard 12au7 input tube and Acme Audio wall duplex. In both cases I found rather dramatic results, but I must admit that I had NOT heard either device without the cryo treatment, therefore my observations are qualified. I can say that the cryo Mullards were some of the BEST sounding input tubes I've ever heard so that must say something of that process, seing that I've never read anything of that vintage Mullard rib-plate's as anything special. I've not explored "cryoing" anything else IMS but you could say I'm a fan.

MikE