All perfectly good theory I suppose, Richard - but I'm afraid it *proves* nothing. You hear what you hear, and I hear what I hear. That's about the only facts of the matter.
I live in a 122 year-old property; I've got not idea what's under the floor (it could be just as substantial as what you have), but in the past when I've used the system downstairs in my lounge on the concrete floor, I can still hear the very marked difference in performance between my gear on and off the Mana supports, and that includes the Techy!
If the PL-71 on your floor is somehow miraculously immune from the effects of vibration (or any other sonically detrimental influences), the only way I'll believe it is to hear it for myself
Also, on pfm, per-Sony-fied seems less than convinced that the carpeted concrete slab was an ideal platform for his LP12 to perform optimally, which tends to disprove the earlier theories put forward in that respect.
Marco.