It's all in the box. Going with a higher order bandpass is not gonna get you a bigger bandwidth than what you already have. Stick with ported with a lower tune or think about going with smaller size subs with a T line. Talk to CSCStang about what he can whip up for you. There's also a folded horn design which has good bandwidth as well. 18's sealed would have good bandwidth but won't be as strong output down low. If it were me, I would go T line for ultimate bandwidth.
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