Have you considered an active sub (including amp)?I'm considering getting one of these to fit discreetly under the passenger seat:http://www.bluespot.co.uk/stock/spkr0_2.asp
What about the fact the frequency doesn't go low enough? Mines sweet FA to me, but you seemed bothered
I think it looks a good unit for the price, it's just fitting it somewhere. I mentioned before, under the seat is the heating vents for the rear passengers. Won't mix well with the amp.
could still mount it in the boot I guess?