I have tried several amplifiers in my system and they all show transformer hum; (there's no hum from the speakers). Clearly this isn't the fault of the particular amp.

The hum often increase after the time the amp is switched on. If music is played at louder volumes, the hum generally gets more pronounced and stays that way even after the volume is reduced.

= What causes this sort of hum?
= Does it harm the amplifier?
= Does it degrade the music? (I haven't noticed this myself.)
= What can be done about it?