My vguitar player has a Marshall tube amplifier, and when it is turned up past three and a half and he mutes the guitar strings, a LOUD humming sound occurs. He's telling me that it is because the AC outlet he's plugged into is not a steady or a bad power supply and causes the noise. Is he full of crap, or is this true? If so, what would cause it and what can you do about it? I know this is a guitar question but I don't feel like joining a guitar forum to ask one question. If anybody has any knowledge on this subject I would really appreciate feedback, thanks.