when you tried the cheater plug, did you plug it into the top plug in the outlet or the bottom plug in the outlet?
This sounds a LOT like ground loop problem -- which is where 2 different grounded devices (your sub amp and another piece of gear) are both grounded and the hum is between the 2 grounded devices. If you plugged the cheater plug into the top outlet, the metal ring on the bottom of the cheater plug would be touching the screw in the middle of the outlet and would be grounding the sub through the screw. Plugging the cheater plug into the bottom plug should keep this from happening.
When the sub is grounded, attaching the rca cable (or any cable) to the rest of your gear would cause it to share the ground with other devices and the hum would be back.
do any other devices in your audo system use 3 pronged plugs?