Regardless of the sound I think you will find that cyro'd tubes don't hold up as well. There are certain things that can be helped out by the process, but tubes are not one of them.
Cryo valves- any experince?
I us 8 Winged C tubes in my Int Tube amp "LAmayro" (a swedisgh design) and they sound very good. There are now Cryod tubes for sale. Any experince with cryo EL34?