6x9s are a pathetic excuse for speakers. Only suited for the parcel shelf of a P platers ford laser.
Don't be shy about it.
Go 3 way dynaudios up front run of 2 massive amps and 3 x 15" DD subs in the boot run off as many monoblocks as you can fit.
- Or the standard option that will cost 10x less, and quite often sound just as good
A decent set of 6.5" splits up front and a 12" sub in the boot. Either run it off a single 4 channel amp, or a small 2 channel for the fronts and a monoblock for the sub.
I generally wouldn't bother with rear speakers. I'd prefer to just spend more money on the better quality fronts.