@murphyb, what you are doing is pushing the woofer so hard the voice coil is hitting it's bump stop. You have two choices. Turn the volume down or get two 12" subwoofers. A single Martin Logan 212 balanced Force subwoofer would do it.
Subwoofer Popping
Can someone help me? 5.1 system. My set-up: Marantz SR6015 receiver. Four Bowers and Wilkins 705 S2's for the R/L and rears and the B&W HTM71 S2 center channel. All speakers set to small. The REL T/9x is my sub. Marantz is set to LFE + Main. Crossover setting on the Marantz is at 80HZ. I have tried for many months to get my settings right on my set-up but it seems no matter what I do my sub often makes a snapping/popping noise when trying to play LFE at a high volume when watching movies with the family. It's as if something is set too high and sending too much power to the sub. What settings do I have wrong? For the life of me I can't figure out how to adjust everything correctly on both the sub (the phase, HI/LOW LEVEL, CROSSOVER, and the .1/LFE LEVER) and the various settings on the receiver (crossover frequency for the REL, subwoofer level adjust,