FWIW here are my personal views (and education as a Ceramic Engineer) on cryoing:
I'm not convinced that cryoing a rectifier tube is beneficial in terms of improved sound quality or not - However I'll remain open minded.
Cryoing got raised on another thread recently and my concern is that, even though the metallurgy theory of cryoing to relieve grain boundary stresses is well understood tubes are made from various materials and metals with differing thermal expansion coefficients. As they cool down and heat up the different materials (glass/metal) will expand and contract at different rates, with the potential to weaken the vacuum seal and shorten its life. I had a Tung Sol cryoed signla tube degas in the past, which makes me wary of spending $$$$ on rare tubes that are cryoed. This will not stop me from playing with cheaper cryoed tubes, such as 6N1P's, were the risk is lower. But with rare 5AR4's and 5U4G's, that have also been around for 50 years, I wouldn't risk the potential risks of cryoing them.
Using the cryo process for musical instruments/race car parts/machine tools (all single materials) is a reliable and well understood process, non of the tube mongers understand the materials science of what they are doing.
I would stick with a good Tung Sol 5U4G/GB or 5AR4, and forget cryoing.
