are the subs touching the back of the seat?... if too close all the sound is being absorbed by the seat back. is the back of the seat open (springs and cusion)? upholster the seat back to eliminate the sponge effect of the open seat back.
do you think i should just go with 10's because the box is about as small as i can make it."with out hitting the magnet aganst the back of the box"or are the mouting depths about the same with a ten.
depending on your budget, 10's would work, or you could do something with maybe four to eight 8" subs. just make sure your current amp can handle the ohm load presenred by the multiple 8's - or buy a new amp(s) - that is what i meant by "depending on your budget".