mmm woofer couch, four fifteens downfiring sealed with the cusions on top, that could be interesting.
I run a single fifteen in a large box with a lot of power behind it, i have found that the larger subs are quite capable of keeping up with the mids and higher frequencies but are often inadiquitely amped. I have 1000 rms to my fifteen and i guarantee i do not use nar half that but i do know that i have impact for days, which is part of speed. I listen to a lot of metal with double kick drum and selecting a sub driver took awhile, at first i just said i will run a car drivewr at four ohms, but most sucked so i chose an adire tempest, mainly on price, but also its pretty damn linear and quick.