6 cu ft and you want 38ish hz? i'll try and cram some numbers tomorrow @ work and see what i can come up with.
--edit
just playing around on the net i ran across this http://www.cerwinvegamobile.com/CVM_enclosures.html that should give you the size port you need along with length of port and gives you a net volume to achieve 40hz. just figure out your numbers for 2 subs and you should be good. also heres another good tool http://www.ajdesigner.com/speaker/index.php <-- sealed/vented/bandpass box building programs. you cant "design" the box with it, but you can fine tune the numbers with it. hope i helped out a little
--edit
just playing around on the net i ran across this http://www.cerwinvegamobile.com/CVM_enclosures.html that should give you the size port you need along with length of port and gives you a net volume to achieve 40hz. just figure out your numbers for 2 subs and you should be good. also heres another good tool http://www.ajdesigner.com/speaker/index.php <-- sealed/vented/bandpass box building programs. you cant "design" the box with it, but you can fine tune the numbers with it. hope i helped out a little