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.

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