Hi Sir Quack,
Your suggested tests are good. Most recordings made before the advent of digital recordings in the early 1980s (and even after, because many studio engineers were suspicious of early digital recorders and continued to use their Studer analog tape machines for years) and transferred to CD will have audible tape hiss.
It will vary from one recording to the next. Sometimes in the re-mastering process systems like No-Noise were used that would greatly reduce tape hiss (some Miles Davis recordings transferred to CD) but those were criticized for lopping off the decay on cymbals, a side effect that was audible if you listened for it. So some engineers stopped using noise-reduction systems during re-mastering.
Anyway, the guy who bought your amps is young and may not realize that plenty of recordings have audible analog noise.
Regards,
Alan
