NickKUK, did you identify and look up the datasheet of the specific Schaffner 6A filter you used ? Did you repeat your 'measurement' at other times during the day? Have you bypassed the filter (or replaced it, such as with a simple IEC socket) and probed the neutral cable again? Did you use a scope (or ?) that is not connected/powered by the mains?
How did you identify that your amp has a 20kHz noise for starters?
I would suggest you could be seeing smoke and mirrors due to your measurements, and the Schaffner filter is just a red-herring.
It's also obvious from the linked article that commentary relates to harmonic voltage levels measured across LISN standardised resistance. Nothing to do with power, but everything to do with related equipment and mains distribution impedances and resonances causing sometimes significant changes in disturbing voltage signals.
How did you identify that your amp has a 20kHz noise for starters?
I would suggest you could be seeing smoke and mirrors due to your measurements, and the Schaffner filter is just a red-herring.
It's also obvious from the linked article that commentary relates to harmonic voltage levels measured across LISN standardised resistance. Nothing to do with power, but everything to do with related equipment and mains distribution impedances and resonances causing sometimes significant changes in disturbing voltage signals.