if u dont wanna buy whole new shit, just change ur head unit, i consider an alpine, they have the best sub control. and when u get that turn ur sub and bass controls to the max. you might clip ur components if u dont have an amp running them, but the sub should bump. and btw theres only a certain amout of air a voice coil can push, so getting gigantic amps wont solve it. if you wanna save some money go to home depot and get some 3/4 sealed wood and some polyurethane and liquid nails and build a custom box to specs. thas what i did, check out my system.
http://www.cardomain.com/memberpage/587607/3 and let me know how ur project comes out