I think the 2 major factors in deciding whether to use footers or not are of course the type of floor you are on and the speakers themselves. A suspended wood floor would definitely require footers of some type regardless of speaker used. On a concrete floor you won’t get near the vibrations but still may benefit from footers. I think this is where the type of speaker you have comes in. Smaller lighter speakers might still be susceptible to vibrations but a heavier speaker with a very robust cabinet I believe would not realize much benefit. I put a dual indicator on top of my speaker with no sound and the needle didn’t move. I then played music including heavy bass edm and the needle move .006-.008 of an inch. Very little vibration in my opinion.
@hilde45
In your test at your friends house what is the jump you are talking about when tapping on one speaker? Was it a jump in sound or did you do a measurement like I did?