This is a ground loop, and often needs to be dealt with on a case by case basis, but in general, can be traced by disconnecting things from the back of the preamp one by one. When the hum stops, take the power cord of whatever was disconnected and try powering it from a different outlet. If the noise is more like a buzz, start by disconnecting cable TV. If it stops, an isolation transformer for the cable line may be required - please contact your cable company or home theater retailer. Depending on their nature, ground loop problems may also be taken care of by using XLR connections (if available) instead of RCA.