I use Pinnovators' sub connection modules and Polk PSW10 subwoofers on all my Sterns. Easy peasy and their volume control/headphone jack panel for the coin door is indispensable too. Unfortunately, the latter isn't available from them (yet) for Spike games.
I typically run the connecting RCA cable out of the cabinet via a screened vent. Just pry the screen up as needed to fish the cable through.