In my opionion, the only way you're going to get the displacement in a sub that you require to keep up with your Four Pi's is to go with an infinate baffle set up. Are you familiar with this term? Also, where in your house is your listening room (basement or upstairs)?

Because a picture is worth a thousand words, here's a few links that might inspire you:

http://home.tampabay.rr.com/kraushome/IB.html

http://t-3.cc/users/audioworx/page100IB-Gallery.html

http://f20.parsimony.net/forum36475/