I think it is caused by timing of the caps charging up on the 12v supply. High current as the caps charge up will make problems on the 12v and the amp can let out a nice belch (ground at different potential / voltage drop??). Fresh high quality caps for 12v filtering on driver/sound board and ensuring the sound board has a good +12v and earth ground would probably help. The SB-300 floats up off the backbox ground. It has one ground return with two wires on a single .156" crimp. One wire goes to the SDB, other to the rectifier board. There can be no/little resistance. There is extra ground pins on the connector if you want to spread things out while repinning the board.
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