Noise. That damned demon you may not even know about...

[DasguteOhr]

no need to measure leakage current or chassis voltage when we can listen to music in both ac polarity (proper and inverted) and choose which sounds better

maybe, but it's about noise&hum (grounding loops). I mainly want to improve that If you don't want that, that's fine. I want to get the best out of what I can.
My tip read a book about it

Amazon.de