Jon, the fact that unplugging the TV audio got rid of the hum indicates that it's most likely a ground loop related to a cable TV input. You can buy one of the cable ground loop isolators. In the meantime, you can try to partially remove the RCA plug so that the center conductor still makes contact and transmits the TV sound, but the outer shield doesn't make contact and carry the hum into your receiver.

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Enjoy the music, not the equipment.