Sorry for the slow in getting back to this. I wonder which system you currently have. I tend to think of a sports car needing small and light stereos. I'd look into adding a DVC 4ohm, 8" sub in a box with amp, maybe 300 watts rms. Polk makes something like that for speakers, not sure what amp to look into, but maybe a small class D can push such power. You could maybe stash a pair in place of some other speakers and skip the box. Or try for a low profile box under the passenger seat, in trunk or just under the rear deck.