I've used a single 8" Kicker Free-Air and also a pair of Audiobahn 8" subwoofers. The results were decent, enough to fill out the low end of the system.
1999 Ford Contour SVT #900/2760 (SOLD)
2005 Mazda 6s
2003.5 Mazdaspeed Protege
1998 Ford Ranger 4x4