Here's what I did:
http://986forum.com/forums/general-discussions/28813-fresh-take-subwoofer-locations.html
This 6" sub has a gigantic magnet and kicks plenty hard for my tastes. Sure, it doesn't shake the pavement, but it
does shake my seat, and the rear view mirror too if you crank it. Powered by an Alpine PDX5 in place of the original amp in the frunk.
That, plus new door and dash speakers and I'm very happy with the sound system.