The Bricasti dac I use does not connect the outer shell of the RCA's and XLR's to the power ground pin on the power cable. So they are only signal grounds.
On my amp, the case, XLR's and power input ground pin are all the same, so there is no chance of having a signal-only "ground" unless my device is a dead end box with magic crystals and does not connect to ground.
Ozzy, have you put your ohmmeter across the rca's of your sub and the power pin of the iec for the power cable? If there is a short, then the RCA's are connected to ground via the power cable. In that case, trying to connect the grounding box to a outlet ground would create a loop.