I do not recommend screwing with your amp settings...... ....but.....
If your amp settings get screwed up, this is what I would do. Do this at your own risk as this has the potential to damage your amp and/or speakers if set too high. But please do not do this unless your amp settings get totally out of wack. Warning, this setup procedure is going to be loud, so warn the people around you.
Step 1: With the game on, take the backglass out and press the volume up or down button on the back of the LCD and bring the pre-amp volume down to 11. It will show on the screen. This is probably set high from the factory, but I found reducing this made the audio quality slightly better.
Step 2: Put the backglass back in.
Step 3: Turn every knob on your amp all the way down until it stops.
Step 4: Go to Music Test in the service menu and select reactor 3 by pressing the start button a few times.
Step 5: On the coin door, press the volume up button until it reaches 11.
Step 6: On the amp, turn the second knob from the left (sub crossover) up about 1/8 turn.
Step 7: On the amp, turn the treble just past half way up.
Step 8: On the amp turn the Bass knob up about 1/8 turn. Keep this low as this is controlling the bass output to the upper speakers and could damage them or the amp if set too high.
Here is where it is going to get loud...
Step 9: On the amp, turn up the sub volume very slowly until just before you hear a bit of distortion. This will be pretty loud.
Step 10: On the amp, turn up the Volume knob slowly until before you hear distortion or you feel that is the loudest you would ever have the machine. This is also the time where adjusting the treble up or down should be done.
Important note! If the amp cuts out during any of these last 2 steps, you went too high with the volume and/or Bass knob. Bring these back down so the amp does not cut out as this could damage it.
Step 11: On the coin door, turn down the volume back to a reasonable level and enjoy.
Again, please do this at your own risk. You can damage your amp and/or speakers messing with this stuff. I am not an expert audiophile or whatever it is called, I am just knowledgeable enough to be dangerous here. Don't blame me or Charlie if you break something.
Let me know if anyone has any better suggestions.