Springs under turntable

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?

Spikes vs. Springs
They are obviously selling something but bring up some interesting observations.

https//youtu.be/dW9-r83IvhI

Your tympanic membrane moves immediately in conjunction with the pressure front created by whatever frequency of sound you care to talk about including bass. This movement occurs immediately and does not wait for a full wavelength to pass. But, it does take a period of time before the sound is registered and interpreted by the cerebral cortex. This probably occurs at the same time for all frequencies. Now, in deep bass it becomes not only hearing but feeling that are involve in assessing the sound. At some point it becomes only feeling if the volume is loud enough to register at all. 
The job of the enclosure is to isolate the woofer's rear from it's front waveforms so they do not cancel and to hold the woofer rigidly in space. Any movement of the woofer distorts the waveform. If the enclosure is sufficiently heavy and stiff it does not matter what it rests on. Unfortunately, that is a very difficult goal to reach so it is always best to anchor the subwoofer to a large immovable object like your house. 
The way I use my subwoofers is significantly different than say millercarbon. I cross over to them much higher at 125 Hz. This is up into the range that can be located. So my subs have to be arranged around the satellites in a symmetrical pattern to maintain a proper image. 
I also have to duplicate the radiation pattern of a line source so that the subs can keep up with the satellites as distance away from the speakers increases. It is certainly true that a bass instrument's higher frequencies and harmonics locate the instrument. The fundamental does not even have to be there. What goes AWOL is the sensation. I can make an EQ preset that chops everything under 40 Hz. Switching back and forth between the normal curve and the 40 Hz chop will not change what you here so much but all the sensation you get being at a live concert will disappear, gone. Those frequencies under 40 Hz are what makes music breath. Unfortunately, it is so easy to corrupt that end of the spectrum with room problems, phase inconsistencies, poor enclosure design and execution, and under powered amplifiers.  But, when you get it right it is a beautiful thing.

Mijostyn's sub array worked for me (I don't have nearly the quality of subs that mijostyn has) primarily because of my room set up did not accomodate sub placements throughout the room. (Thanks mijostyn). I also cross pretty high based on the simple premise that I wanted a speaker (sub) to "take" the "whole" instrument. A tweak that might work for those using an array is sending left channel signals to the left side subs of the array and right to the right. On many recordings, it's clear, by hearing and feel, that that double bass is clearly on the left of center. YMMV.

@mitch2  thanks for posting Townshend link, I use them with Cube speakers and imo they improve clarity.

I came across this which may be of interest as well

https://youtu.be/5ihzvD3urc4