I skimmed through the first 17 pages of this thread, reading maybe half of the posts.
I often find that it is helpful to start by applying some simple logic to the question, and to allow the logic to drive the investigation.
Here, the following seems appropriate:
In order for an external power conditioner to improve the SQ of an amplifier or other audio component, the two following statements must both be true:
1. Distortion routinely exists on the mains, which many typical audio components on their own cannot deal with effectively.
2. External power conditioning is able to correct the problem by presenting the audio component with a more perfect mains power supply.
The proper way to conduct this investigation is to start by performing a thorough study of the distortions that are routinely encountered on the mains. Once that has been done, the next step would be to study the capability of typical home audio components to deal with the kinds of distortions that are routinely encountered. After those first two steps have been done properly, it is indicated to undertake a study of the capabilities of individual, external power conditioners.
That's mainly what I wanted to say. But I also want to point out something that I do not think has been made adequately clear, at least not in the first 17 pages of the thread.
The power supply filter in any audio component is tasked to filter out all AC leaving a pure DC signal. The nature of any filter that can do this is such that the higher the frequency of the noise or distortion component, the more effective the filtering will be. It follows directly that assuming that the frequency components of noise and distortion are weaker in amplitude than the fundamental (60 Hz or 50Hz), that if there remains an audible artifact in the audio output of the component, the artifact will be ripple at the fundamental mains frequency (60 Hz or 120 Hz, or 50 Hz or 100 Hz). It follows directly that, with that same assumption, for any audio component with a power supply filter that is adequate to suppress the ripple to a level below the threshold of audibility, noise and distortion on the mains is not a concern. In other words, for any audio component that is capable of adequate suppression of the fundamental mains frequency, the only possible way that an external power conditioner can be beneficial (other than surge suppression) is the case where the frequency components of noise and distortion on the mains are stronger in amplitude than the fundamental mains frequency.
If perchance there is consensus that the preceding paragraph is correct, then it would seem obvious that the investigation, of the question of whether external power conditioners are useful for more than surge protection, reduces almost entirely to the question of how common it is for frequency components of noise and distortion on power mains to be stronger in amplitude than the fundamental. It seems to me that this is what this big question reduces to. Maybe someone has provided information of this sort in the remaining 18 pages that I haven't yet read. I suppose I'm going to have to read the rest of it.