I am planning to upgrade my Bose speakers but keep my OEM HU in my coupe. I am relatively satisfied with my bass but want more mids and highs. I was wondering if I should just replace the front 6.5", or front 6.5" + rear 6.5", or all 6 speakers. I figure I can get away with not replacing the 6x9 since the bass is decent. How much difference would it it make by switching 1 set of 6.5" vs 2 sets. Thanks

 

Personally I'd equate this to putting lipstick on a pig. Your pig looks nicer, but in the end it's just a pig with lipstick on. I guess if I was into this sorta thing I'd just replace the front component set with as nice a set as would be possible, disconnect the rear 6.5s all together and put some bass blockers on the rear 6x9s. This would, hopefully, shift the sound stage more appropriately towards the front, kill the "rear fill" garbage, and increase your SQ by using a nice set of front components. Unfortunately, unless you are willing to go with an aftermarket amp, your front component SQ is going to be limited.

 

Here is a running link that may answer your question. I'm in the process of doing the same thing as well, replacing the front & rear speakers and possibly replacing the 6x9s in the near future.
https://g35driver.com/forums/audio-video-electronics/75337-what-s-best-speaker-upgrade-power-oem-hu.html