Al, did I miss something?No, I think you covered it well. The only thing I would add is to re-emphasize the point I had made that some of the spurious difference frequencies that may be created, especially as a result of intermodulation of recorded hiss or music and system-generated hiss in the speakers or other analog parts of the system, may be at frequencies that are more audible to us than the original frequencies.
With respect to the purely analog parts of the system, I'll qualify my comments by saying that I have no particular quantitative feel for how audibly significant noise intermodulation may typically be. (The significance of jitter at the point where digital is converted to analog is well established, of course). But what I am saying is that if the subjective perception of background blackness can in fact be improved by reduction of noise that is essentially inaudible when nothing is being played, the explanations that Kijanki and I have offered are the only ones that come to mind.
Best regards,
-- Al