I'm looking into making an enclosure for two SX18s. RE suggests 6.5 ft^3 per sub tuned to 32hz. Thats like 13 ft^3 for two 18s. How many SQIN of port area should I use for this box? On the DD site they use 16 SQIN of port per cubic foot of box volume. Should I use that and then tune it to 32hz?