If you haven't done this yet I have a couple other thoughts.
The 6x8's you are using will give you a better bass response for sure, so while you're installing them may I recommend putting some sound deadener in the door sub cavities. It will get rid of the echoing type bass and make it more precise and hit with more clarity. The door subs aren't terrible and most experts say either keep them or change them out after you replace the other things if you are on a budget. The rear speakers in the back? I wouldn't worry with them. I changed mine simply because I had an extra set of speakers, but while they are full range speakers most of the sound is in the front sound stage and the back is mostly filler. So don't worry with them, for what they do they aren't terrible either.
I sound proofed my doors and those cavities when I swapped out speakers and the difference was remarkable. But if you just do the door sub cavities it will make an improvement.
Last edited by RetiredSFC 97; 10-31-2014 at 01:18 PM.