What is your desired effect? My interpretation of your setup is that the 6x9's are there for boot open running rather than up front sound. Is this correct?
I don't think you really need an 8" as you have the 12" for bass.
What you want is either some good 6.5" coaxials or better still some 6.5" components. You've got enough bass so what you need is clear mid and top which 6x9's have never really been able to do.
What freq are the 12's x-over'd at?
An 8" would be good (there are some REALLY nice but terrible expensive 8" mids) but then you'd really need a 4" mid as well and then you are getting into active territory and more amps.
What makes/models are your fronts?