Jack, a few thoughts. Try a cheater plug on the power cord and reverse the polarity. Try taking the bolts out that mount the transformer and put rubber o-rings between the bottom of the transformer and the top of the amp around the bolts holding it in. This will make the trannie sit on the o-rings and not be in direct contact with the top of the amp. You said the shop tightened the bolts, but usually it is a different thing to tighten the transformer bells than it is to tighten the transformer to the top of the amp. Either can cause hum, and both should be addressed. Good luck.
You ask if this is normal, but don't give the make or model number of the amp. Perhaps some psychic audiophiles can give you guidance on that one.
You ask if this is normal, but don't give the make or model number of the amp. Perhaps some psychic audiophiles can give you guidance on that one.