i took the liberty of doing a bit of homework on your box design, the overall box volume because of the vent legnth is over 4 cubic ft, your right the tuning frequency is about 25 hz, at least 5 hz lower than the f3 of the box and the woofers fs, both of which arent at all desirable, i just thought that i would give you a heads up to try and save you some time and $$$$, 2.8 cubic ft sealed gross internal would be great for that woofer, and if you wanted to port it 3.7 gross internal with a port 3x12x17.3 would give you an f3 arounf 30 and a tuning frequency of 40 hz, neither of the designs inclide bracing so you would have to take that into consideration yourself, good luck with whichever design you choose