Aftermarket subwoofers behind the seats?

[ljd-924se]

people have squeezed smaller diameter subs behind the seats; i believe these JL's are 8", but JL also makes a 6" midbass woofer.

[Perfectlap]

I think you look under Boxsters on the Cardomain.com site you'll see a yellow Boxster that has allot of money dumped into the audio. Try sending him an email, I'm sure he's tried allot of different set ups.
Those bass shakers that go under the seats are often used on roadsters. More for those who want to feel the Bass instead of hearing it.