So- something mysterious exists on the AC line that can't be filtered out??
The best power conditioner made that I've ever seen was made by Elgar. It could regulate line voltage and deliver a sine wave with less than 0.5%THD (in AC lines its the 5th harmonic that is the most troublesome). It could do this without limiting current and their larger ones had current capacity of about 28 Amps @ 117 Volts. These are large units designed for 24/7 operation in commercial/industrial applications.
The 5th harmonic can affect power transformer efficiency (and can cause them to become noisy). But if the power supply is regulated, an efficiency loss in the transformer isn't going to do anything.
I have no idea about Elgar AC power conditioner but I think the highly regulated sinewave line voltage with low THD does not necessary mean good sound. you know industry produced high feedback high power soildstate amplifiers with low THD but the sound never was good. for example the Mark Levinson 33 amplifier is ideal voltage source low THD but ML 33 never sounded as good as low feedback amplifiers like Lamm/dartzeel/Vitus/CH .
the problem of most AC filters is not only current limit and even most high current AC filters are not good. you know parallel AC filters do not limit currents but their sound is not good.
I have paid for many Isolation transformers, AC line filters (both series and parallel) , AC regenerators, voltage stabilizers and ...
the only right way to me is AC regeneration , (Kevin owner/designer of Living voice also use AC Regenerators).
if you have a full range (extended low bass) and very dynamic system then the result of line filters is:
decreasing dynamics for cleaner sound. I do not like to trade off dynamics for more clean sound and i prefer to find a better way.
two ac filters that i did not test are Ayre L-5xe and Lessloss 640x filter and i would like to test those filters before my AC regenerators.
the AC problem is the biggest challenge to me.
I hope a good company like CH Precision produce a 30KVA AC Regenerators for Audio Systems.