Thanks all for your responses. I have learned some things. Right now, I plan to take the amp to the dealer and see if it hums in his set up. Interestingly, I took the ST 100 out of my system and put an ST 90 (the model previous to the ST 100) in its place---and NO HUM. Suggests to me its not
dirty
power,
but more likely a bad transformer. I assume the dealer will hear the hum also and then it will be off to the factory for repair.
Regards, Rich
Regards, Rich