Ditto,
I put an 8" Sub, one that uses THE SMALLEST suggested cu. in. box volume. A couple better amps and some nice door speakers. Sounds LOUD, CLEAR and THUMPS! Don't waste your money trying to bass out of the center speakers....
My 8" sub is also in the passenger footwell area. Tucked up against the left leg area. STLL PLENTY of room for passengers feet.
Gerald