Cryogenic basics-
Some materials become superconducting when passing current at cryogenic temperatures (some beryllium alloys). They in effect lose all of their resistance- (e.g. 10 to the -29th power ohms ! ). Do your E=I/R calcs on that one !
But ONLY at cryo temperatures- e.g. minus 270 degrees F.
Some materials become brittle at cryo temperatures e.g. carbon steel.
Some do quite well, e.g. stainless steel, copper)
Cryo capable metals do undergo micro grain structure re-alignment when submerged in liquid nitrogen. Some re-alignment does become permanent, Whether it changes or improves the sound while conducting an audio signal is not proven.
Because of the large range of temperature differential between room temp and cryo temp (e.g. 340 degrees) the materials are subjected to a relatively large amount of thermal contraction. A risk for audio electronics durability is that an assembly of different materials that are closely connected may experience some thermal related stress because the materials contract at different mounts per degree of temperature change. Without doing any calculations I would be hesitant using cryo treated vacuum tubes that have a sealed assembly of steel pins, mica insulators and a glass vacuum vessel. The materials will contract at different rates and the stresses involved could cause material fractures.