Help, my system has suddenly developed a mains hum: I noticed this at the weekend. Over the last week I have changed the mains cables to my receiver, Sky + Box, and LCD tv, as well as the mains cable to my sub. My sub cable is now plugged into a different mains block from the rest of my components: this is the only 'positional' change to the leads going into the components. I removed the high level sub connection last night which reduced the sum markedly but it is still there in the background. I have read somewhere that the sub and the receiver should be plugged into the same mains block such that they share the same earth: this would make some sense and could explain why this problem has suddenly materialised. Any thoughts anyone?