Originally Posted by Mojo
Could be a leaky filter capacitor in the power supply. You could be more sure of that by cutting open a spare RCA cable, shorting the wires together and connecting the mess to the sub jack. That eliminates any possibility of airborne noise getting in via the sub's RCA jack. It is a high impedance input after all.

Can you please elaborate/simplify on this?
Are you suggesting cutting an end off of the cable, then joining the two wires inside together, then plug this shorted device into the rca jack on the sub?