I did an install a while back using 2 pioneer ts-w306c 12" subs... I build quite a few boxes and settled with some outrageous box. 3 cu ft^3 NET @ 28hz (per sub)... that way over the recommended of i think 1.75 (1.5 for yours), but that was the magical box that wanged like no other... it was being powered off a hifonics tx1505d... i must admit i was impressed with the outcome... the customer was extremely satisfied especially at only being $350
I must admit those are some pretty dandy subs... very trust worthy