I asked almost this same question a while back.

The Firepower and Black Knight I used to play at the arcade in the 80's had a box speaker attached to the top of the back box and sounded awsome.

I posted if anyone knew about this upgrade, as I would love to do it to my current Firepower and Black Knight, and basically got no response....and most said it was never done.

I later came across a picture of me in the arcade, with both Firepower and Black Knight in the background with these speakers on top of the back box.

Would still love to know what is needed for this upgrade.

You could always put a car amp/eq in it to run the bb speakers and an external powered sub under the game.

They probably just hooked it up parallel with the existing speaker. If it was 8 ohm too it kicked up the power going to both speakers.

FP has an 8 ohm speaker, but you might find them hard to find locally or expensive. 4 ohm is much more common now. However, you can run two 4 ohm in series and still match the sound board amp's impedance.

Firepower only has a single cabinet speaker.

I believe Black Knight has a single cabinet speaker, 6" 8 Ohm. Part Express http://www.parts-express.com has a ton of speakers that have these specs in a wide price range. I really don't know if replacing the speaker will produce the results you're after but just about anything is better then the stock speaker.

Bill