This board has a logic ground return and a 43v to 12v regulator return. When the two grounds are at different levels it lets out a nasty hum. Ultimately this is a likely connector issue at the 20pin rectifier, top right SDB connector, sound board connector, or the ground braid between the cabinet and head is not connected.

The easy fix is usually to unscrew the sound board from the grounded backbox mount and let it float up off the grounded screw down points. The proper fix is likely repin the ground connector with resistance.

I have never seen that voltage regulator board before. I wonder if switch mode noise is getting into the 43v - 12v regulator circuit. Do you have a stock SDB or different SDB to try? I will say an EZSBC on a stock bally driver board absolutely can introduce switcher noise into some Bally sound boards (cheap squeaker i think it was).

This sound board is one of my most popular boards. There are ton of them in service over the last 5-6 years, there is not a general design issue with them that should make them hum. If anything the grounding and filtering should be better. They are dead silent in my games (assuming the connectors are good otherwise nasty ground hum with bad connectors).

I can check, repair/replace the sound board if you want to send it back to me.