I agree, I've had a few machines that were upgraded in the speaker department. Did they sound better? A little.

But simply using polk audio subs did more for less, can be used on multiple pins at once and you get to keep them when you sell a pin.

It's a no brainer IMO.

Not sure if you have ever heard a real dedicated subwoofer enclosure, but it is most likely 20 times more effective than any sub woofer that uses a pinball machine as a speaker box and your machine doesn't rattle.

When subwoofers and enclosures are designed, they get specific ports and airspace.
I don't know all the intricacies of building a speaker box (about the only time someone builds a sub box is for a custom install in a car), but anyone that has had a powerful car stereo knows you don't use something like a lower pinball cabinet as a box, it's more like a sieve and has too many loose pieces.
Subwoofer boxes are typically very heavy because they need to be solid, nothing to shake or vibrate.

Try one out, you will fine the the high's and mid's on the machine itself are enough and the polk sub (which is adjustable) provides the missing bass plus some.

My batman forever was amazing with a polk sub.

For the record, I think nobody's wife like's an external sub.