I put the back box speakers from pinball pro in mine. Ran the sub wires from Greg's kit down the back box and out a vent hole in the base of the cabinet. Then took the wires and connected to the right side speaker in on my Polk audio 12" powered sub. I turned the volume knob that comes with the kit (controls the bass/treble of the back box speakers) all the way up (more treble - to the right). It rocks! I do not have the factor sub from stern hooked up any longer. Some people still use the factor sub by jumping off of it and running wire out to the powered sub. I did not want to do that... It's probably safe - so many do it. I didn't take that chance. Make sure your powered sub has the regular speaker wire hook ups. (Commonly called high speaker bypass) Mine is crazy loud now and a blast to play with the bass up and the shaker motor on high!

I would agree with Bischt in the past - however since I hooked up the external powered sub on acdc that’s the only way for me in the future. Of course that’s just my personal preference.