On some amps such as Krells, you need to plug the XLR inputs with a shorting strap or else you will get hum when running single ended. Otherwise, check out the basics such as cable dressing etc. If you have a Comcast or cable box connected to the system in any way, disconnect the RG6 Coax (main cable feed) going into the box and see if the hum goes away. If it does, simply lift the ground with an inline filter and you should be good. You may also want to check the cable terminations on the single-ended cables as well as the RCA inputs and outputs on the connected gear. Loose terminations and cold or loose solder joints can induce hum. Just a few thoughts.....
