MC, You wrote, "Audio is full of complex concepts and this is a big one: human beings DO NOT hear all frequencies the same." Yes, that's right; it's called the "Fletcher-Munson Curve", and you didn't invent it. It's why once upon a time a conventional receiver or integrated amplifier had a "Loudness" contour knob on its front panel.
Then you wrote, "So if there's no location information, and moving the subs around by
feet all over the room never alters any bass detail, how can it possibly
affect (nonexistent) bass detail if a spring lets a sub move a
millimeter? Rhetorical question. It can't."
Here you are conflating location information with distortion. If the sub moves in response to a low frequency audio signal, that means that some of the energy in the signal was lost in doing the work necessary to move the speaker. This could cause an aberrant presentation of the musical signal at certain frequencies, due to loss of energy at certain frequencies. That phenomenon is a form of distortion. So, in my opinion, it is best to anchor a subwoofer as firmly as possible in the listening environment. You can argue the other side of that question, but not using the rationale presented in your last post. You'll have to do better. I don't disagree with your bit about bass location. But I will turn your conclusion around; if the springs do nothing to affect bass detail or a sense of bass location, then why use springs at all?