I am actually about to write up a huge "how to" on this. But in short, you can use this:
http://www.amazon.com/Legacy-LWF8X-Series-Audio-Subwoofer/dp/B000060NXK/ref=sr_1_5

For your sterns, you will need to cross them over. This is a 6db 100 hz low pass inductor:
http://www.parts-express.com/pe/pshowdetl.cfm?PartNumber=255-286&DID=7

You wire it inline on the positive side (one side goes to the + term on the speaker, other goes to the striped speaker wire.

But it's still not enough. You now need to be able to increase the bass volume. This is done by lowering the cabinet speakers. You can either wire in an L-pad vol control such as this:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=260-248

I tested these three things in my iron man compared to flipper fidelity sub set up and could not tell the difference.

Actually the speakers that turbo20lbs pasted will also work. I think that for the money I use the amazon one plus I have prime so 2nd day shipping is free.

The 2030 amp used in the Stern SAM system is a 4 ohm capable amp. You can use a 4 ohm sub if you wish. I believe the FF subs are 4 ohms.

Sorry, I haven't forgotten. Give me a few more days. I want to make sure all the info is right when it comes to best for the prices and want to make sure it all applies to XMEN LE that should be here tomorrow or Thursday.

I did do a big write up on ACDC just now:
http://pinside.com/pinball/forum/topic/massively-improve-your-acdc-premiumle-sound-under-80