Just got the car back this weekend with new system installed. We were shooting for sound quality/clarity not boom. I think we hit the nail on the head.
Alpine CDA-9813 runs to an Audio Control EQL then to a JL 300/4 amp. The XR650 components up front are driven by one side of the 300/4 at 75 watts per side. The rear TR650 coaxials are run off the radio and toned way down to provide rear fill only. I must say that they are important to the soundstage - we tried it with and without. The other side of the 300/4 is bridged on (1 )10W3v2 in a sealed box in the factory Bose location. We tried a 8W7 but could not get the right dimensions for a box in that area. The sub is firing towards the roof and is anchored down quite well. It sounds better than we expected in the factory location. Time delay on the radio helped that some.
Throw in some dynamatt on the doors, etc. and you've got yourself a system.
This is not a system where someone will hear you coming from a mile away. But it is one that will put a huge smile on you face if you love a clean accurate system.
Overall, I am very pleased with this setup.
Hope this may help someone . . . .

 

I will be doing something very similar but keeping the Bose HU and putting XR650 Woofers in the rear stock location for rear fill. I am strongly considering the 300/4 amp and have not decided on a sub setup yet, but like the idea that you have come up with. I am simply aiming for better SQ with reasonable volume capabilities.

 

Its funny cause i have the exact same system minus the back speakers and it sounds very good..

 

To me the rears add a depth to the soundstage. It does sound good without, but to me sounds better with. I have the fader almost all the way towards the front. You must really concentrate to hear them - but they do make a difference.
I wanted to just do the XR components up front with nothing in the rear. But my friend/installer convinced me to put some in. I'm glad I did.