I also thought it was the RCA's, but they are shielded and do not introduce interference in other setups in the same location. Further, the noise is still present even when I remove the RCAs from the equation (i.e., everything is plugged in EXCEPT for the RCAs). I suppose it could be the power cable or a ground loop, but wouldn't that be in both speakers?
The noise does not seem to change when I move the speakers around, but I have not moved them into another room.