Al, I don't mind; and, thanks. I think your comment "Which of those two kinds of amplifier/speaker interactions results in the most correct acoustic output from the speaker is a separate question, however." is particularly valuable and relates to another issue that you bring up, perspective.
As I said previously I tend to let those more technically astute explain these interactions, but after a lot of years in this hobby I am convinced that part of the reason that some of us "are at loggerheads" (besides ego and stubbornness) is that there are still aspects of sound and music and the perception of those that the technical doesn't fully explain. I would like to offer some thoughts about this from my perspective.
In my experience, and almost without exception, systems assembled with the stated goal of "linearity" don't sound the way that live music sounds. More times than not the result is sound that is tipped up in the highs and lean through the midrange compared to the sound of acoustic instruments. The argument that there is too much variability in the sound of live to establish a benchmark is nonsense; there is much more that is consistent than there is that is different. Most of music takes place in the midrange and most audiophile systems sound too lean in this range without enough fullness and image density particularly in the lower midrange. I can't remember how often I have heard comments about a system lacking bass when there really was no true bass content in the music and what the listener was missing was the appropriate fullness in the lower midrange and, perhaps, upper bass that gives music much of its power. "Linear" highs will often result in upper partial information that is not well integrated with the fundamental frequency and add excessive "presence" to the highs.
"Brightness" is an interesting descriptive term in that it is often confused with "texture". I have heard many systems (or recordings) that are described as "bright" which I would describe as harsh or grainy but definitely not bright. In fact I have heard systems that are dark and harsh (usually ss based) and others that are bright and overly smooth (usually tube based). Incorrect texture is what I hear oftentimes with ss amps driving electrostats and not necessarily excessive brightness (even when the sound is too lean, giving the illusion of brightness), and that is the main reason that tubes driving electrostats usually sound more natural to me: the fullness and image density of the midrange and the proper integration of harmonics with their fundamentals creating a texture and clarity that is much closer to the sound of live acoustic instruments even if, in absolute terms, the sound does not measure as "linear"; whatever that is.
