I have for sale a Clarion SRV250 compact powered subwoofer.
Amazon.com : Clarion Mobile Electronics SRV250 Powered Subwoofer : Vehicle Subwoofer Systems : Car Electronics
It's brand new in box, never installed.
This is essentially a 150W amp in an 8" ported enclosure, with a wired remote for adjusting the level and frequency. It is very compact, very simple to connect, and would add the perfect amount of bass to an open car like the Boxster without rattling your bones.
I have sold my car before I got a chance to install this, but I planned to install it in the back of the passenger seat, hidden away in the "scalloped" contour of the seat and rear firewall. It might fit in other places too if you are inventive.
I'm asking $105 shipped to conus.