I always wanted do do a setup like yours but could never design a solid box.I did the design myself using http://www.carstereo.com/help/Articles.cfm
But I did call 80inches and he gave me a very good pointers, also Gmonkey gave me some numbers to start with for size of ported/sealed sections... Most 4th orders have the ported section where the front of the subs is, he told me to flip the subs so the motor is in the ported section which will help cool the subs. I'm glad I did it that way now I can play them longer without being worried.
I always wanted do do a setup like yours but could never design a solid box.
Could you explain more on how you design it using the calculators in the link?
Also,how does it perform on higher bass freg?