It may be some kind of grounding issue in your electrical somewhere. It has happened to me as well, where one amp is perfectly silent, and another amp buzzes / hums like crazy through the speakers. Some amps are sensitive, some not.
A quick check to see if there is a grounding loop / problem is by inserting a ground defeating plug into your outlet, then plug your amp into it. If the hum is gone, you have a grounding issue.
A quick check to see if there is a grounding loop / problem is by inserting a ground defeating plug into your outlet, then plug your amp into it. If the hum is gone, you have a grounding issue.