I can't advise you to connect the safety ground from your AC receptacle to the RCA jack. I'd expect the RCA jack is not connected to safety ground internally but rather what is called "raised ground". Raised ground usually has a resistor or capacitors or MOVs or even all three connected to ground.

It's a pain, but you may want to try disconnecting everything from the receiver and then sequentially plugging everything back in while you listen for the hum.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated