well the problem with this set up is that the sub (if rated for free air) will usually only be able to run half the rms because there is really no air chamber and the sub will flex more but not put out good sound since it will flex farther and not have the normal recoil as a sub in a box would. if you are not careful with this setup you can blow the sub easily. not a very practical idea unless you build an enclosure underneath it which has a nice air tight seal so then u would basically have a sub in a box. you can try it but more than likely you will not get what you are looking for in a good system.
1994 SSE, 180 t-stat, gucci accented interior, suede headliner, eclipse avx5000 headunit, two 7.8" sunvisor screens, two 7" headrest screens, dvd player / tv tuner, infinity kappa perfect 5.1 components, infinity 12.1 perfect in the trunk, 2 rockford fosgate 350s powers, infinity refrence 7541a, 0 gauge audiobahn high flow wiring, duralast gold battery, sound dampened trunk and doors, xenon-white headlights, 2 15" and 1 20" red neons shine on the back doors, 2 3" and 6 12" blacklights lighting up the whole interior, red leds in the vents, red optx el neon pedals, red streetglow headlight leds, clear corners, pilot seven color angel eyes, blaze seven color underlighting kit, viper 791 xv.