This topic links very well with this one:
First of all, thanks to @VLS again for introducing me to to the IDEAL SureTest 61-164 circuit analyzer https://www.amazon.com/Ideal-Industries-INC-Circuit-Analyzer/dp/B08NPPGKKW which retails for some $400 This device measures power line phase, voltage, impedance, leakage to ground, and peak...
www.whatsbestforum.com
And that package is what I have been working on/with for some time now.
The Ideal meausurement is useful to get the basics right, large diameter cable to the audio system and comparing power cords and selecting the better ones. It also reveals the best power lines if you're using existing ones.
For me, now that's a full stop for the purpose of the Ideal meter (Eye on "max peak (virtual/calculated) current" whether the system will ever need it or not.
Next there is the THD, Using the big fluke and staring at awful THD in voltage, worse THD in current and shocking current shapes. What to do with it and what is audible here. I did some trials, but don't try this yourself (with my EE background I did this in a safe way). I looked at the harmonics and the 5th was popping out. LC series calculations result in a notch filter that would drastically take out that fifth. I had the right HUGE and safe capacitor for this in combination with the approx value of the inductor. It worked and the 5th was almost gone. Next I listened to the the difference with/without and yes there is a significant difference, but since this was an experimental set up and drawing a significant amount of extra current shorting that 5th I don't use it at this moment.
There are professional THD reducing units available that even filter out most harmonics in a dynamic way (constant THD measuring and adaptive filtering these out)..huge expensive and mostly 3-phase. Not for now for me.
Third area, noise. THD is part of it but stretching the spectrum way up you might be looking at MHz and beyond. Filtering, two options, the in-line with LC etc but being in line it will increase the impedance of the power supply, good for noise reduction (as done to meet EMC requirements) but poor for high peak current demanding equipment.
The other way of filtering, seems like a free lunch, the Shunyata approach, apparent cables going through black boxes, assuming it's kind of piezo/HF-absorbing, might work, never saw spectrum measurements of this...but if this is a restriction of the noise, it is still a restriction in the power supply line.
The step I took to at least make a next step, big diameter cabling and a trial with a 3kVA balanced isolation transformer. Listening tests with/without....tremendous difference, better or worse? Well, without a doubt better with, blind tested and pointed out, my wife included. Measurments, EMI meter factor 5 reduction (if I remember correctly) on 1 meter and hardly a difference on the other.
For now, pause and enjoy the current configuration, however.....want to try the Furutech NCF plug with the expensive "Shunyata" similar approach in mind. Future, dynamic THD filtering and COSQ correction would be great but not for now..