a superbly detailed, dual opposed 15" build, by a great contributor that we don't see much of anymore.
nice build, utilizing one of my favorite sub 15" drivers, the JBL-GTI-MKI 15's. The magnetic design of these drivers is fascinating, and they incorporate a significant amount of research from the giant of the industry. So many drivers come and go, these on the other hand have been availed to the enthusiast for a while.
Also, examine Bosso's contributions. He feels strongly that the 15" drive unit is the sweet spot for the DIY execution of a subwoofer. Whether you pursue his approaches or not, the contributions he's made to this community's collective knowledge are greatly appreciated.
Best of luck