Mike:
The site shows the dimensions of your powered subwoofer at 15-3/4"W x 3-
11/16"H x 9-1/16"D.
That means it would fit under your driver seat with room to spare.

I would strongly advise installing a dynamat, fatmat, or other butyl
rubber sheet goods for sound insulation at least in the seat podium area
to help your sub out.

And it does help a lot (even though it's scary) to cut out the front panel
of the podium allowing access from the front of the seat area.  It looks
like that can be an option for you as you have a remote control for the
sub.

My Infinity BasslinkX is a bit bigger and would barely fit under the back
bench.  But, like you, I wanted to limit punishment to the passengers so I
mounted it to the back base of the cabs passenger seat.  It doesnt swivel
anyway and I can still snap in my jump seat as needed.  I even sewed (dont
tell anyone I sew) a little cloth bag for it so that it's not obvious what
it is.

Feel free to holler if you have questions.

Brent
www.weidefamily.net/vanagon/