Need help choosing the right box design and subs

[Skeezix the Cat]

I drive a GMC Savana and it is very difficult to get any amount of SPL with so much airspace that I have had to get bigger subs that take more power, which in my experience tend to prefer lower frequencies.  I really want to get two Platform 5 18s to replace my one Platform 4, but I also want to get more of that 38hz tuned sub sound.  That just seems like it would be wasteful on these subs because I'd never make use of the excursion capability at those tunings.  Low frequencies are fun, but virtually all my music sounds five times better on a 38hz system than my 32.  I've been brainstorming off and on and for a while decided I would use Bigass ports and adjust them if I'm feeling like playing some real earthquake stuff but I think there are better ways.  I know there are some box designs that feature great frequency extension but at the cost of overall SPL, and also are a little too complex for my woodworking skill.  Could I build a dual chamber box, where one sub is tuned differently from the other?  Then make up for the SPL with raw power. 

Alternatively, for a normal ported box I would go with 36hz as a happy medium.  Are there better 18" options for that tuning than platform 5s?