This is an interesting thread, and I'm surprised the topic hasn't been discussed before (to my knowledge). The main speakers in my system are Vandersteen 3A Signatures, which are capable of clean response down to about 26-28 Hz. The floor of my living is medium carpet over wood, and sits above a 3-foot crawl space. I was concerned about the amount of bass resonance, so I decided to mount each speaker on top of piece of MDF that had been a sheet of DexDamp on the underside (a resonance control material similar to DynaMat). I removed the spike feet from the speakers and used 6 Vibrapods between the speaker and the MDF panel. I found that it substantially reduced the speakers of their warmth, and seemed to shift the tonal balance toward the middle frequences. Several good audiophile friends confirmed the same thing, and they thought the change was definitely NOT for the better. So, I went back to the original setup and was much happier. This proved to be an interesting experiment, however, because it showed me that decoupling a speaker from the floor is not always a good thing to do. It may be significant that the Vandy speakers use a Sound Anchor stand that has 3 spike feet, and they sound much better when properly coupled to the floor. Of course, your ProAc's may react differently.
