Rolling KT150s in Primaluna hp integrated

as I'm not sure the cryo process is a good idea for tubes but that's maybe just me

It's not, all it does is weaken the tube and cause vacuum leakage.
When you freeze especially this cold, different materials, they contract at different rates, then expand back at different rates.
 
And in a tube you have glass, high temp plastic or bakelite if nos, metal pins, different metals inside nickel in nos all sorts of different materials.

Cheers George   

George maybe you're right but then again maybe if the tube goes through the cryogenic process one may well end up with a better tube for having survived. This would be providing nothing before and after changes with regard to measurements and vacuum leakage. All I can say with any certainty is not so much about whether it works or not only that the cryoed tubes sounded better, CLEARLY. I have read some of the debate and controversy concerning this issue, seems more nay than yeah at least with regard to tubes. In my case it very well may be that there was more miles on my original KT150s than I realized and that putting in fresh tubes made the difference and it had nothing to do with the cryogenic process. I have to find out the parameters to test all the subjects (KT150s) on my Hickcock to see if this might be the reason or at least give me a clue. 

If your amp can run 6L6 tubes, it would be interesting to try some vintage tubes of this type.  While they don't deliver much power, I really like the sound of that type of tube.  I also like 350s, but at close to $1,000 a tube, that experiment probably will not get off the ground.

Larryi,  If only we were billionaires!  I'd finance that experiment now!

The Tung Sol 7581A is a very very good tube.  

FWIW, I briefly replaced the Tung-Sol KT120's in the CMII, and rolled in Genelex Gold Lion KT88's.  Perhaps they were slightly sweeter, but I missed the greater authoritativeness of the 120's.