You could run free air subs through the 6x9 holes? Or add a small box around the 8" subs too. Problem is you'd have to make sure to decouple the rear deck from everything so that it doesn't rattle the hell out of your vehicle. It's a tough situation.
But I'm like you...so I'll be adding a hybrid audio I6SW in the front of my vehicle. Scott Buwalda had PWK speaker design a box for one of his competition vehicles using the 6.5" sub. Pretty neat.
It's the second picture here...
https://www.google.com/search?q=hybrid+audio+i6sw&client=firefox-a&hs=qDD&rls=org.mozilla:en-USfficial&channel=fflb&source=lnms&tbm=isch&sa=X&ei= rwe6UZOkMPTB4AOip4BQ&ved=0CAkQ_AUoAQ&biw=1280&bih= 841
I'm going to use their vented recommendation unless I decide to try and design a tapped horn enclosure.
EDIT: I believe the 6.5" subs replaced two 15" subs? And everybody has said they're MUCH louder than the 15's they had.
