I have 2 behringer 18s that sound fantastic and shake everything but fall off pretty sharply around 40hz. I also have 2 pyramid WX-15X subs in generic boxes that struggle to hit 30hz. I've been using winisdpro to run scenarios with the 15s (as well as other 15s just for comparison) and the perfectly tuned box for my 15s is 12.4 cubic feet each!!! That makes them pretty much flat from 20hz up but there is no way I can have 2 boxes that size. What am I missing here? How are all these other subs hitting so low with such small enclosures? The specs for the WXs are. Fs(hz) 30.0, Qms 4.57, Qes 0.64, Qts 0.56, SPL 88.6, Vas 3.973. Unfortunately, I don't understand what these numbers mean and am simply relying on the software listed above. Any help would be greatly appreciated.