if youre tryin to do what i think you want to do, youre not goin to use the box you have now. what you want is a box with two separate chambers, with each of the chambers having its own port. you'll need to get a massive alternator and about 3 more batteries. as far as moving air, they dont have to be 15's, what youre looking for is high excursion woofers. look at something like resonant engineering, thats a much better choice. if you want to do a hair trick, youre going to end up building wall with 4 12's in there, and buy stock in dynamat or something, cuz youll need a LOT of insulation. you'll need to push like 10,000watt per woofer cone to get to where you want to be. but it is doable, cept it wont come cheap

I just wish my car was as fast as it was loud. people tell me i should use my sub to push it, it might go faster.