Short version of what I did:
- New backbox speakers like those Pyles http://www.parts-express.com/pyle-plg42-4-coaxial-speaker-pair--267-717 There are other models.. anything in $20 range should be fine.
- New cabinet sub - Pyramid 8" http://www.parts-express.com/pyramid-wx85x-8-white-poly-woofer--290-272
- Get a pair of these equalizer/amplifiers http://www.amazon.com/BOSS-AVA1404-Amplified-Equalizer-Subwoofer/dp/B0002OI39Y
- Then finally an old PC power supply to use as a +12V source. Or you can simply buy a good (10a+) 12V power supply.
Install the amps and power supply in the cab. I use the service outlet to power the setup.
Bring the game's speaker output from the backbox down to the cab, split it to L/R channels and connect it to speaker-level input on one of the amps. Bring that amp's output back up to the speakers (you'll only use the front or rear set of output - use the fader on this amp to control volume level of backbox by dialing away from the output you chose).
Repeat for the cab sub - game's sub input to speaker level input on the other amp (just using one channel L or R - I don't think these amps can use both channels on one speaker, and one is plenty for that Pyramid anyway). Run output of one speaker to Pyramid sub. Use fader to control volume, although in all setups I've done this stays at full and it's the backbox that usually needs turned down.
Now use the equalizers to adjust the sound just how you like it. Kill the bass on the backbox amp and kill the highs on the cab amp; adjust highs/mids for what sounds best in that particular game. Remember that the game sounds different when closed up, so adjust and play a few games, then fine-tune adjustment.
To really kick it, add an external sub and use the cab sub as mid-bass and the external for the booms.