Is it possible to modify do to fit a pair of 6x9 speakers instead of a pair of 6.5??

 

Anything is possible.

You will be possibly cutting into the door panel for the speaker to fit. Some custom work is necessary.

Why do you want to put 6x9's in the rear?

 

I have a set of infinity kappa 3way 6x9's that are the best sounding rear speakers I've ever heard. I'm getting the kappa perfect components for the front and want to match it perfectly with this pair of 6x9's that I don't wanna get rid of. I wonder if the modification would be too much of a pain in the a$$ but it would be lovely if I can. How much do you think it would cost to mod??

 

Why don't you try just a front stage for now? The rear speakers are just for "fill".
A good front image is what you want since thats where you are sitting.

I have a pair of Focal 6.5"'s in the rear for fill. It is just my preference for sound but the front image is very good.

Maybe some other more seasoned audio installers here on "Driver" could help you out.

 

you would probably get better results using 6.5" speakers. 6x9s are not that great...even though the cone area is larger because of the oval shape they do not reproduce bass very well and tend to distort along the uneven cone shape. round speakers are symetrical and do not have this distortion pattern.