you can just get one kick ass powerful 10" sub... might take up 30cm of box room, and you can get it installed in a corner (sub facing out) and thatll give you plenty of bass.
besides that i can only suggest going to an audio store and talkin to them. take your car and they should look at it all for ya.
when i bought my car, it had a small alpine sub to save space. the shop moved it and played with my amp, got double as much bass
