For those interested in a more comprehensive upgrade for their door speakers, Kicker makes a set of component speakers (separate tweeter and midrange woofer) with the same three-hole mount as the full-range KS600, model KS60.2. You have to take the door panels off to replace the tweeter and mount the crossover, but while you're at it, you can vastly improve midbass response by laying some vibration damper on the backside of the panels...
Stock speakers are 4 ohm front, 6 ohm rear. You shouldn't have any problems replacing the rears with 4 ohm subwoofers (I did a few months ago, and have been running them off the stock amplifier up until this week). 6 ohms is a not-very-common load, and most amps can handle a range of impedances anyway (usually, down to 4 ohms stereo/2 ohms mono for multi-channel amps).