The cryo process for tubes has been experimented with to a great extent by Bruce Wenger of BWS Consulting. He has a web site, http://www.bws-tube.com/
If the information about cryo treatment is not listed at the site, you can ask via e-mail. Bruce does modifications, can repair older gear and is an excellent source for many of the tubes we audiophiles are looking for. He is very sensitive to the sound of things, not just an engineering viewpoint. In other words you can talk about the sound of tubes with him much like you can with Andy Bauwman at Vintage. Hope this helps.