i have HEARD, i have HEARD that depending on the amplifier that you are using such items as high current equipment on 120v ac mains have a tendency to hum. that probably doesn't make sense at all, but it is just what i have read. something happens that the ac current drops and somehow the power company can install something to fix it. if this isn't a vague answer, then show me one lol, but it is just what i have read and heard about. since it probably will elude you, then i assume it isn't the problem.