I would strongly encourage a sub. These come in all forms as you know from tubes to individual box speaker combos. When we say sub don't think of that punk kid with a lowered honda with altezza lights thumping down the street at 11pm at night. Think of a more defined bass line and a more realistic thump from the bass drum. Do a comparison. I'm not sure if your into Dave Mathews Band or not. Pop one of there CDs in the player in your car and listen on your stock system w\ out a sub. Then find someone with a mosdest subwoofer setup such as 1 10" sub or a single 12. The music seems to come alive.
I totally aggree with the comments made above about 6X9s. They make a lot of noise but will not give you a satisfying defined sound. Whatever 6.5" component set you decide to buy to replace your front speaker, use the exact same set for the rear. Now your system is acoustically matched and balanced all the way around.
By the way guys. What is the deepest 6.5" woofer that will fit in our door pod? I don't want the window hitting it. Also how do you get the panel off to remove the door speakers??
Thanks
Brad