I've added Polk PSW10 powered subwoofers to nearly all my Sterns, my Houdini, and my JJP POTC.
All Sterns were connected to backbox driver board via a Pinnovators sub connection module (also recommend their coin door audio control units available for SPIKE or earlier games). POTC sub was connected directly into the game's CPU's audio output using a Y-splitter to allow continued connection of headphone jack on front of game as well. Houdini sub was connected directly to sub speaker connections provided in backbox.
