I have had 2 episodes with hum. Cable TV coaxial lines are notorious for carrying hum into a stereo system. Try disconnecting coax lines and see if this is the source of your hum. A small isolation transformer put in the coaxial cable eliminated this hum. Some components with 3 prong AC plugs can cause hum in my experience. If you can find the offending component then a cheater plug (one with 3 slots on one side and 2 spades on the other side) may do the trick. This happened to me with a turntable, the cheater plug eliminated the hum.
