Sounds like you might have a slight dround loop problem as when the receiver is off the hum goes away, maybe try plugging the receiver and sub into different outlets, also disconnect any other equipment from the receiver one at a time to see if one of those components is back feeding through the receiver.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70