Been enjoying my EP500 for a year now w/no issues. All of a sudden when I turn it on now, I get a very low level faint hum. When I turn the Yamaha Receiver on the hum intensifies quite a bit. I know this sounds like a ground loop, but NOTHING has changed w/my equipment and all connections seem firm? When I unplug the RCA cable from the receiver to the sub, but leave the power on the sub, the hum completely disappears? What has happened?

Thanks guys!

M60 VP150 2x QS8 EP500 HK 7300 DENON DVD-3910 Sony 60" SXRD