Thanks for your reply. Instead of the single 18" I'm considering building a box for the 10's and tuning to the fs of the subs which is 29hz. Much cheaper route and might work, but might not. I'll find out.
Since posting my intro here I went ahead and acquired 4 8" soundqubed midbass drivers for my doors. I'm going this route despite reading here and there that its not a good idea. I'm going to run them at a narrow 80hz-250Hz and see how that sounds. Maybe it will rattle the s**t out of my doors I'll try and sound deaden as much as I can. I listen to lots of types of music but its metal like Slayer that is missing too much midbass on my system. The bass guitar is so important to that music and I can barely hear it. The 10" subs sound pretty good above 80Hz compared to 15's but I'm still wanting to turn my crossover on them down to 80Hz.
I'm not sure whats in the dash, probably 3's they sound really good. I may have to add some 5.25" drivers somewhere, but I doubt it. I'm not sure how well a couple 3's are going to balance out with the 4 8" mids. Ill find out.
Have a good day.