Hey again!!!
Just googled for some "20hz subwoofer designs" and found this, a bit of surprise for me looking at this was, both front & rear waves of each driver combine at its final path before the generic path (marked in red), two different waves, no cancellation will there be, (I'm wholly new to this)???
Plus, I'm glad if anyone of you'll be able to find me online the dimensions of this, I don't remember its source, just an image search that was...