The speakers behind the seats are already longer throw bass units and are ported already in that 'hollow' behind the steel wall with the fuel tank.

You can probably just replace those with higher end units without going bigger in size. The bass response is wonderfull on those.

The main speakers in the door should also provide plenty of sub-bass response as well unless you did the 'capacitor mod'.

If you want your car to 'jack-hammer' others on the road... get a big ported box and suffer without the trunk space.

Cheers!