So I have a buick with the monsoon factory setup that I will be rewiring to be powered off an aftermarket head unit. The rear originally had 4" for highs and a 6X9" for subwoofers. I have since removed the 6X9s because I added a sub. So what size speaker should I put for my rear fill. I obviously don't need all 4 just for rear fill. I was thinking changing the 6X9 hole to either a 5.25 or a 6.5. I would think that the 6.5 would get better mid bass and if I got a two way I could get some highs out it as well. Is this my best option to do what I am after?