Not trying to talk you out of putting in a subwoofer, but thought I'd share my experience from my Scion Xb. I managed to fit a pair of 8" speakers in the rear quarter panels and the sound was quite good. I still thought I might want a subwoofer, so I took my Klipsch powered subwoofer from my home and hooked it up in the back of the car to get an idea of what kind of improvement I'd get. The Klipsch was a 200 watt powered subwoofer with level controls, crossover frequency controls and phase shift.
What I found was that for most music and most music sources, the subwoofer didn't add any noticeable difference to sound quality. The only time I heard a difference (and it was quite a difference) was with electronica or rap/hip-hop that had a lot of electronic bass and drums and then only if it was from a CD or other digital source. For rock and roll, heavy metal, classical, and more, the subwoofer didn't make a difference.
I'm providing this info because you can get an 8" speaker in the front doors fairly easily. If you add good vibration damping to the door, you'll have plenty of bass... except if you listen to electronica/hip hop/rap.