Then change the box you have your W6's in instead of buying a new sub. You've got more than enough surface area to get loud and low with what you have. Try a ported box tuned lower or even try a larger sealed box.
Either one of my HC 18"s once they are reconed. They are low end monsters //content.invisioncic.com/y282845/emoticons/naughty.gif.94359f346c0f1259df8038d60b41863e.gif ...