No, I do not have an explanation. However, there are other data points: I only reexamined this whole idea because of a comment I read in the tweak thread, or Panzerholz thread or Tang's thread about pneumatic isolation robbing a system of harmonics. I don't remember the exact quote. This made me think of the differences I heard between Rockitman's TechDAS AF1 and his AS2000. One of the major differences is the presentation of harmonic content. So I experimented. Only after the positive result with the Sinks, did I try it with the Vibraplanes.
The effect was very similar between the two pneumatic devices. I was surprised. This suggests that it might be something other than interaction with the turntable suspension or something to do with the ceiling. Other data points: The effect also occurs when deflating the Vibraplanes under my two amplifiers. Finally, before even playing with the Vibraplanes, I took one Townshend Sink over to Al M's house to try under his preamp. Bingo, same effect, as I suspected. He was surprised and heard the same thing.
No, Fransisco, I think something else is going on, but I don't know what it is. It is possible that it simply damps the component and robs the reproduction of harmonic content, precisely as suggested in a post on the subject comparing these platforms to Panzerholz.
Now, other people may have other results or the same and prefer them. As I told Marc, I first liked the Vibraplanes because they create a more focused, bold, stark, contrasty sound, particularly in the bass region. This can be nice and different and attractive because it is a change. I'm not suggesting anyone else will have the same results and I am certainly not telling others to deflate their pneumatic isolation platforms. This is what I am hearing in my system.