I would ditch the 6x9's personally. Just make a plate for the size speakers your using out of thin plywood. I've got 6x9's in now and can't stand them. Will be replacing them with 5 1/4" infinity reference speakers. I find the 6x9's are causing way too much rattling.
you can go with 6.5 components in the back as well if you make a mounting plate out of some material (probably wood) and you have a subwoofer system capable of filling in the mid-low bass.
I actually have this setup on my solara (now my sister's) because the previous owner was either a jazz or classical fan. before adding the subs that are currently in there, most music sounded pretty horrendous without any mid-bass at all. with the 2x10" subwoofers that are now in the car, it sounds much much better. I tuned the subs to produce as much mid-bass as possible (and also why I went with 10" subs for quick mid-bass response) and it sounds great most of the time. Only to the untrained ear does certain frequencies seem lacking. I would replace the 2 RF 10s for competitive subs with better response, but it's not my car. But just know that if you lose your 6x9s, to properly fill in the midbass, you must have something that's adequate to fill the void- and that usually means spending money on good components.
Akward, There is a local radio station that plays jazz on fridays at night and I have found that what they play has more bass in it than most other music other than rap, and even then the jazz goes lower.
i'm using Polk Momos for the speakers- i actually bought 2 sets of 6.5's and a set of 6x9s with the weaved soft carbon fiber cones, single 12" polk momo sub with an aluminum cone and sealed spec'd plexy-box, 2 memphis studio amplifiers, with monster time correct speaker cables.. off my stock radio.
I say it all depends, I personally have two pioneer 6.5s in the front and two pioneer 6x9s in the back, they were powered by a clarion amp. and had two 10s in the back pioneer premier amps. the 6x9s had way to much mid-bass, but the amp kinda made up for that because it was setup for voice on the interior speakers, since the 10s had so much bass, the rear couldnt be heard. so i ended up installing two big tweeters in the back.
Its all up to the driver, i love my bass but i need the vocal :lol: sometimes anyways