Shouldn't they be a half wavelength over the floor? I would imagine if you want your bass to propagate predictably. Now what is the wavelength of 20 HZ hehe.
As for the violinist, she is a performer, and am sure she doesn't like coughing sneezing and hacking during the quiet parts of her solo. So is 'tuned' to it.
Unless I was using the best of tape recorders , I always could hear (or imagine to hear) flutter in piano or similar pieces. The invention of CDs was a boon to me.
Unless I use the finest of CD players, I can always hear jitter(just kidding)