I recently got the Epic Master package, but the subwoofer has a hum coming from what appears to be the built-in amplifier. As soon as I turn the power switch on, a fairly low level audible hum begins. I can hear it easily from about 6ft away. I have tried plugging the sub in a different AC outlet in another room (with nothing else connected), and I get the same hum. Anyone have ideas as to what could cause this? Or how I can alleviate this?