In working out the earthing, I can see ways to improve the PreAmp topology to incorporate the earthing. My eye keeps wandering to the spare piece of Vero board on the PowerAmp that would be just the right size for the PreAmp and it would be right close to the input of the PA.
So the earthing area of design is coming to a close (for now) but it looks like a rebuild of the PreAmp is coming.
I get tantalisingly close to having something to hear and then another step comes along and pushes it back. A little frustrating.