i have 2 12"s downfired in a vinyl wrapped box with a rear exit port tuned to 40 hz, sub faces sit about 3 to 4 inches from floor and still achieved 3 cubes without it being oversized. a 15 would be dificult in fitment considering width. Also with the srs airbag modules underneath, i put a few layers of sound mat over them, no airbags in the face so far. soooo much more posibilities in the center console vs under the rear seat.