People run 8"s in the rear deck all the time. That's the substitute for actually dropping a sub box in. It's going to give you an honest bit of bass that 6x9s wont give you. The coax in the front are already going to be giving you your mids and highs. If I was doing a build like this, that's how I'd do it. But i'm not. Instead I have 4 12"s and simply unplugged my rear speakers. Just to tickle my fancy, while they were still plugged in, I faded the music all the way to the front, heard almost no change. Faded all the way to the back, tons of change.
Like cameron said up there a "bit" as in, not enough to be worth it. You'll get REAL bass by running 8s, and REAL everything by running 8s combined with some quality mids and highs.
