I've got a Kenwood KSC-WA62RC compact powered sub in the rear passenger
footwell of my Celica - it would probably fit under the seat if the
seats weren't mounted quite as low.

I'd recommend it - it doesn't make the panels of the car vibrate, but
it does mean you can hear bass guitar when driving down the motorway.
It really fills the bottom end in nicely, and gives the main speakers
less work to do.

S.