I've had a lot of separate discussions on this topic, so I figured I'd just take the time to post it all in one place. In short, this will be a guide that will allow you to build and add a cheap sub to your WPC game. This is for pre DCS games. DCS and WPC95 games would be a different (easier) guide. This is also solely for B/W pre DCS games. These are unique in that they have a single amplifier. Personally, I have found this to result in a better sound than flipper fidelity or pinball pro provides for about half the cost.
Ok, it's a bit complex. But here goes.
Williams wiring and speaker theory
Pre DCS WPC games use a single amplifier. This is a 4 ohm capable amp. However, Williams obviously had no talent or brains when it came to audio engineering or even the slightest clue when it came to wiring. In order to get 3 speakers working well and keep the impedance into acceptable ranges, you have to wire some of the speakers in series, and others in parallel. If your impedance drops too low, you blow up your amplifier. If your impedance is too high, your amp loses its ability to power the speakers. Therefore, in an optimal setting, we want to run the speakers at exactly 4 ohms.
Williams provides 3 speakers: 2 backbox are 4 ohm. 1 cabinet which is either 4 ohm or 8 ohm. They then wired it in series / parallel as follows: amp + goes to cabinet -. Cabinet - goes to left (5.25") +. left 5.25" - goes to amp -. Then they jumpered the left speaker + and - to the right speaker. Unfortunately, this results in very little bass and overall bad sound.
Ideally, what we need to do is add a decent but cheap 8" subwoofer to the cabinet. It'll need to be 8 ohms. Then we will cross it over so it only produces the bass tones. We will allow the backbox speakers to work as the midrange and tweeter sounds. These are 4 ohm speakers, so if we wire these in series, they will also be 8 ohms. Then we will wire the 8 ohm load of backbox and 8 ohm load of the subwoofer to a single 4 ohm load resulting in some decent bass and balanced sound.
Here is what we need:
1) 8mh (or so) inductor, $14: http://www.parts-express.com/pe/showdetl.cfm?partnumber=266-576
2) Cheap but very efficient 8" sub $21.50: amazon.com link »
3) Some speaker wire. You can probably rip out and use the wire already in the machine.
Now, the inductor is easy to wire. Attach one end of the inductor to the + on the subwoofer. It doesn't matter which end. Pretend the other end of the inductor is the + of the subwoofer and use it to attach the speaker wire. I usually just tie strap the inductor to the speaker, but you can attach it to the cabinet or wheverever so it doesnt rattle around.
Now for the system wiring you need to do this:
1) Amp + run to inductor lead.
2) Amp + run also to backbox left speaker +
3) Amp - run to subwoofer -.
4) Amp - run also to backbox right speaker -.
5) Run one wire from backbox left - to backbox right +
You are done. You should find you have some decent bass sound as well as plenty of volume.
To attach the 8" to the cabinet, just get a hammer and use the back end to pry out the 6" speaker that came with the game as well as the wood spacer that the speaker is attached to. Then use some #6 or #8 3/4" screws and screw the speaker to the bottom of the cabinet centered over the hole.
Optionally, an l-pad could be wired in line on the backbox to allow further volume adjustments and bass balance. This adds some complexity to the wiring and in my opinion, isn't needed. http://www.parts-express.com/pe/showdetl.cfm?partnumber=260-248
DCS games (includes wpc95):
For those that may ask, there is a simpler solution for DCS and WPC 95 games. Simply swap the cabinet speaker with an efficient 4ohm sub. You don't even need a crossover because the cabinet speaker is already electronically crossed over at 300hz. There are two amps in the DCS games. This allows you to only worry about the subwoofer swap without backbox impedance and rewiring. If the cabinet speaker you choose is efficient enough, you can be done. But if not, you can add the L-Pad above or maybe just add a 20 ohm resistor (http://www.parts-express.com/pe/showdetl.cfm?partnumber=017-20) to lower the backbox volume.
If you choose to add the resistor, you would desolder the two wires that go to the 5.25" left speaker on the backbox. One goes to the amp, one goes to the right speaker. Separate the wires. Solder the resistor to the + lug of the 5.25" speaker. Solder the wire that goes to the + of the right speaker also to the + lug of the 5.25" speaker. Solder the wire that came from the amp to the other end of the resistor. That's it.
Finally, for DCS games you'll want to choose a decent and efficient sub. The wh88 will work pretty well as it is extremely efficient. However, it's 8 ohm so it takes some power from the amp away in impedance. I recommend using one of these MCM 10" speakers: http://www.mcmelectronics.com/product/55-3211. They are 95db efficient so you may not need any resistors if you use this. You will lose some volume form the small 6" cutout on your cabinet but it's fine. If you choose this option, your entire cost for the DCS addon is around $15 shipped.