from the info you sent me i got....
each "chamber" has a gross of 4.7cu' assuming that these are 15's (since you stated that each one has about 4cu' so) .5cu' for displacement which gives us 4.2cu'. 4.2cu' tuned to 35hz requires a 6" port to be 10.4" long before port displacement, and 11" long after port displacement. you did not tell if the port displacement was figured in the ~4cu' or not so i did it both with and with out...if each chamber was 4cu' then the numbers would be slightly longer.
as always, i recommend that you get a second opinion to check my work as well.