Just wanted to post my results as well which confirms this is a ground loop issue.
I had bad crackling/hum in my R&B speakers very noticeable in attract and running diags especially during lamp and flasher tests.
Changed out VR1 & VR2 on the sound board. The crackling was reduced - likely because of my busted VR1, but noise was still there.
Changed all the caps on the sound board - noise was still there.
Don't like nylon screws so instead got nylon washers and put one each on both sides of the sound board at the four mounting screws - 8 total. Speaker noise gone.
The problem now is the buzzing Cherry display. I tried a Vishay I have that is partially gassed and the machine is eerily quiet. Except for the buzzing the Cherry works fine so I'll live with the buzz for now.
I should also mention that I have an XPin PS installed so if you have a different config YMMV.
So for about $.25 and 10 min work you can significantly reduce or even eliminate that speaker hum.
Thanks for all the constructive discussion in this thread.