Nutz, it all comes down to how you are using the system. It sounds to me that you are NOT trying to win any SPL competitions or trying to impress anyone with a system that can be heard from 4 blocks away. In my 14 years of installing car audio and video systems, I have learned that everyone has their own agendas. Running your rear 6x9's off of the factory deck will not equal poor sound unless you turn your headunit volume up past 3/4 which is around where you will experience clipping. But since you are using an amp to power the fronts and the sub, you shouldn't need to turn the headunit up that loud to get the volume that suits your listening needs. There is no need put the extra burden on the amp of running the 6x9's if you feel that the rears are only for fill (which i completely agree with). This is of course just MY opinion and there are people that will disagree with me. Good luck and have fun with whatever you may decide to do.