I have done tubes, makes a very nice difference in retrieval of inner detail and smoothness.I havent done trans. by themselves and compared the differences, I have experimented with whole stuffed PCB's and cryo made a positive difference on those.
My son is building a gain clone, I think I'll have him build 2, 1 cryo'd and one not and compare the differences, I post results here in a month or so.
Regards,
Lee