@big_greg , you are certainly not lacking in the subwoofer department. The two 13.5s I assume are JL Audios. What subwoofer uses two 15" drivers?
Next time you are thinking about a new preamp you should check out the Trinnov Amethyst and the DEQX Pre 4 or 8. Digital subwoofer crossovers and time alignment are built in. You can get rid of the rumble and still feel the bass.