I have one factory marine sub but I also have 2 car subs that have had no problems through 2 boats. I've had good luck with car head units and amps as well
Fiberglass resign is what i sealed my last box with.
For all those with car audio subs are all you subs tucked in the storage compartments? I am thinking about adding a 10" or 12" sub underneath the bow filler cusion. I have seen some boxes made and they don't take up storage and still have a place for the extra sub.
I have had Alpine Type R's, Massive CW-12, MTX, and a few other random subs. None significantly better than others. The JL's and Kicker Squares provide a slight advantage in cone size.