Well, after removing the Panasonics FR, things improved (audio started passing), but still distorted and noisy.
So I decided to follow your advice and check the psu:
Removed reservoir caps, they measured perfect, but checking the VRs, found one rail down to 0v!
Could not find solder bridges - but legs were soldered in parallell to board, and traces were VERY close.
So I scratched between the legs with paper and a screw driver. And the short disappeared!
The VR came back delivering healthy 15v to the rail, and the compressor works as it should. Quite a journey!
Perhaps the movement / bending exerted on the VRs when removing the board caused a short... or an invisible solder bridge. I do not know, but either way you were spot on pointing at a problem in the power supply.
Ultimately it was a great learning experience with a good outcome, thank you all for the heads up!