I had an Outlaw 755 that had an intermittent transformer hum. I spoke with Outlaw Tech Support twice about the problem and received the same answer each time -- "your power company either has a low or high voltage problem and their is nothing we can do about the problem." I returned the unit under warranty. Outlaw's claim was that their specification states that the power input must be 115 volts + or - 3%. Well, neither 110 volts nor 120 volts is within that range and I believe that most power systems will see one of those voltages at some time and by Outlaw's own addmission you can expect intermittent transformer hum.

BTW transformer hum is not a ground loop problem.