In the LARGER you have THREE drivers, one Active 12", one active 15" and one Passive 15".
In effect they ALL are blended to act as a single driver over the FR of the SUB.
Brian, often times blends drivers in ways other designers do not. He has (for lack of a better term) a "magnetizer", which allows him to control the level of magnetic strength of the magnets to the drivers. He has somehow developed a system of magnetization that helps him to use this blending of drivers.
When this is added to the very fine adjustments of adjusting the putty mass you have a SUB that can have exceptionally low distortion in a 3 driver system due to the ability to control the synchronization between the drivers and their actions and reactions to each other.