I have identified the boards, re-seated all connectors, pushed on socketed chips - fuses all look and test good. Speakers have a very loud hum/buzz when starting up but it mellows out slightly as it's running, but takes several minutes.

I'm going to start testing power output on PSU and PS AUX boards. I've not found anything on a cursory search of the System 3 or this game to point me in the right direction, but I'm thinking ground issue or power or busted diode?

I checked the grounding strap, that was good - took out the resistor board, reflowed solder, same issue. Took out the auxillary PSU, reflowed solder on the audio amp chips, the connector, and bridge rectifier diodes, and that seemed to fix the sound hum totally. Now I've got a wierd issue with the vertical kick up in the lower right of the playfield - stays up during game play - not sure why, that then stops the ball from launching back onto the playfield until power cycle.

This was because of some toasted transistors on the solenoid board - verified by swapping out with a replacement. Oh the joys of pinball!