I've found the best way is just add an external sub and place underneath the game. Run wires through back, attach alligator clips and done. I would not invest in another sub to be place in cabinet, I don't think the cabinets are properly suited to give quality sound with a free air sub. When you're done with game, simply remove the wires and move to the next one.
These seem to be real popular with the pinball community.
https://www.amazon.com/Polk-10-Inch-Powered-Subwoofer-Single/dp/B0002KVQBA/ref=sr_1_1