The WPC pinball cabinet is so large that it acts like an infinite baffle box. I modeled it as a 300 liter leaky enclosure; treating the head as a port makes little difference. I tried various models and speakers, but they all came out looking rather like the chart below.
The conclusion is that a speaker with a higher SPL and a lower Fs is the best choice. I normally use a Pioneer plastic coned 8" woofer that I get from Parts Express for about $20. In Creature, I simply wired it in parallel with a pair of car speakers in the head. Results in a bit of a mismatch with the volume control, as full volume is at 12. More than that, the amp starts its thermal cutouts. In Monster Bash, I added a 15 ohm ceramic resistor in series with the head speakers to balance them up.
If you use a lower efficiency speaker, or you want an easy way to add a balance control, you can add a cheap car amp powered from the coin door.
You might need to do some rattle work...
Speaker Response Chart