Hi Lee
It would be helpful if you would say what presets cause the problem (this was why I asked).
The meter's default settings is to show inputs, so you will have to read the manual to see how to change them to outputs. I would need to know if they show the problem.
The mixer wiring could cause problems if you were creating a feedback loop. Best think to do is to remove everything apart from piano-h8000-output. Then see what happens. Also, if possible, try different inputs and outputs to see if it makes a difference.
A bad mains supply could indeed cause any number of problems. Depends on how bad it it.
