I think you're spot on with the earth loop. Thinking about it what they were doing was star-point grounding. So heavy power output stage currents don't modulate low voltage supplies. This must be why the grounds are separate as you've pointed out.
I'll have to dig out my BH4000 from long term hibernation sfter reading this. All the electroytics must be near shot now. It did power on without smoke 10years back.
SJM.