After measuring AC voltage inputs to the board and it is found that over voltage is being provided by the main transformer, I think it is best to correct the driving voltage rather than hack the board with the incorrect amp.

From my perspective the easiest way to correct the problem is to add a transformer that provides the correct voltage. Mount it and wire it in.

This allows changing the sound board in the future without facing the same problem.

http://www.mpja.com/24V-4A-Center-Tapped-12-0-12-Transformer/productinfo/27845+TR/

With machine on:
Set your meter to measure AC volts.
Place one meter probe to any ground point.
Place the other probe to one of the sound board fuses, either end and measure.
Place the other probe to the other fuse, either end and measure.
What voltages did you read on each of the fuses?