Just to throw this out there, my work car has an AB XD-8 ported behind the driver seat. The tactile feel is tremendous, I find that the little 8 that close and wedged behind the seat is some of the best bass I've experienced. It is pretty flat and the tactile, well that makes a world of difference from listening from further back.
Good luck on your build.
Where the Governor is on a steak, the cops are on the take, and the welcoming committee is a sawed-off shotgun!