Maybe those 15" MB quart subs that are really cheap would work for you. I think that some one on here is running them in 4 cubes sealed and says he is gonna put them in 3 cubes sealed. He gave rave reviews on them.
A pair of IDmax is gonna be the loudest and best sounding in my opinion.
However 4 12" IDQ sealed would beat pretty hard and soudn good doing it.