I will read better later (at work now, alas), just skimmed. From what I saw, the table holding the monitor is not a great platform and may influence the test results.
I will note that many years ago I took measurements trying various feet under a few speakers. They were all floor-standing models as I recall, including a couple of Magnepans, Quad ESLs, B&W 801 (original, this was ca. 1982~1984 timeframe), a pair of Polks popular at the time, and a few others I've forgotten (*). The floor was concrete with commercial carpet (thin with limited foam beneath). I measured the impulse response, do not recall frequency response showed much, but you could clearly see changes in the impulse response on a 'scope. However, listening tests did not show significant change with various feet (spiked, several flavors of isolators, and in one case bolted a pair of Magnepan's feet to the floor). I also measured Magnepan and Quad (I think) panels with added stiffeners to brace them, same results (measured differences were not readily if at all heard). The biggest difference by far was in the bass, which makes sense given relative mass and such, and that does appear to jive with Ethan's measurements though my foggy memory is that I saw more difference in impulse response than he measured.
That was long before things like Stillpoints and all the high-tech isolation devices of today, be interesting to repeat the experiment.
Interesting - Don
p.s. Measuring other components we did see the effect of vibration on some tube gear, less (virtually none) on SS, and of course all kinds of stuff out of a turntable playing a silent groove.
(*) In college I was lucky enough to work for several high-end stores, and one in particular had an owner who loved running all sorts of experiments. We spent many evenings and weekends with other local 'philes and engineers setting up all sorts of things, and often roped local audiophiles in for various listening tests. One of my biggest regrets is that the mass of notes, data, and various papers I helped create and co-write during that time has been lost through the years. Along with a number of manuals for my stereo system from that vintage. I am convinced we either lost some boxes in a move, they were thrown out by accident (my wife saves newspapers and a box with that data and a few papers on top may have been tossed in the belief it was simply a box of old newspapers), or is lurking in one of several black holes of storage around the house and shed.