I was actually quite surprised when I was listening to a few test songs, the 6.5s have a decent amount of midbass and bass. The doors are sealed very well from the factory.
The plan was to put the 8s on the other two channels of the leviathan, with the same crossover settings from the 6.5s. I was going to do the ac10s, but looking at the response the 8s, it can be crossed over much higher. I realize to add more speakers is to add complications like cancellation. But atm I'm testing out a few drivers, and if need be I will only use an 8 or 10 in place of the 6.5 and the morel 378 can be crossed over basically wherever I need it to be with any of those woofers. So far I am impressed with the quality of aurum cantus. The 6.5s on the leviathan at 8ohms don't seem to reach overexcursion even when I turn off the highpass filter completely. If you have a better solution/recomendation, I am open to change. Better solution is to not mix drivers. It will suck. Without more info, I can't help.
Umm, no. Faital blows donkey IB. Terrible idea. Stop reading wherever you are getting your ideas from.