This sounds like a mechanical vibration issue. Try this please. When the amp is on and humming lift it from the shelf so it is in you hands not touching the shelf.Is there a change? Next with it on the shelf put your hands on the amp and press on it, like hug it lol, not enough to crash things but to see if pressure on the chassis changes the sound/hum. From your descriptions it is possible the transformer has loosened or in some way is physically vibrating. What brand and model are you listening to? You say it always has had the issue. How old is it? A solution may be as easy as opening the cover and tightening whatever keeps the transformer in place. Then too is could be a defective transformer. GL
