Looking at the back of the Behringer Feedback Destroyer - it accepts dual XLR inputs and outputs dual XLR, for (I'm assuming) balanced audio. I've never used one of these before, so let me see if I can get it right -
From the Receiver: (single sub out connection)
I would need an unbalanced RCA to Y-adapter, and two seperate RCA to XLR adapters into the left and right XLR inputs on the unit.
From the BFD:
Left and Right XLR output to RCA adapter cable to either a.) dual subs (left and right), or b.) into a single sub.
I'm basing this on the assumption that the unit tweaks the line level signal between the processor and the sub's amp.
Let me know if I'm way off base on this or if I've got the idea.