Mississauga, ON
I bought the same 10" Polk sub for my Tron Pro based on posts I saw here on pinside. It will never leave my Tron.
I left the backbox speakers intact and hooked up my external sub directly to the wires previously going to my factory cab speaker. I was going to clip on but decided to pull the factory Stern speaker (25W) right out. I mean why bother leaving it there? I installed it instead into my DM replacing the crappy cab speaker it had (an improvement actually).
I've played a friend's Tron Le with an installed internal sub. It sounded great and did not shake the glass. However, IMO the bass is not as good as my external sub. Mine 'feels' better 
The external sub is matched to its box while an internal sub works with the cabinet air space which is probably not ideal for that driver. With the external sub it is easy to manipulate the crossover point (within what is already being sent) as well as the sub volume all the while still allowing the pinball volume control to manage overall game volume including sub.
Price may be the deciding factor because both setups give good bass. I waited for a half off sale. Up here in Canada a FF Stern woofer kit would cost $45 to ship. Local shipping of the external sub was only $13 (and arrived in two days)... Overall price difference to me was negligible.
