I've determined that the Clarion 7" SRV303 powered subwoofer (can be found for $150) will fit under the front passanger seat. I did a cardboard mock-up to test it. It will not fit well at all under the SE driver's seat due to the adjustments, wires, brackets, etc.
This subwoofer will not blow the doors off, but people familiar with it say it does a nice job of giving better low end, and should serve to round out the 10 speaker JBL system.
What I'm wondering is - how much bass impact I might loose by not putting it under the driver's seat?
Clarion SRV303 Powered SubWoofer