He has a sub, since he owns an Si.check out the Maxxbass processor, do a google search on it, ive heard the theories behind it, and it makes sense, could work wonders. dunno the price tho
edit: oh, there would be some wiring behind it. as for the difficulty of the wiring, i dont know, but without a sub, i think this is your only way...
to the OP i think the only way is to get better subs and a new amp.