Most audio equipment comes fitted with rubber feet. That's just as well as rubber is a great isolator, offering hundreds of times better isolation than spikes. Whether audio equipment need isolation is another matter entirely.
Loudspeakers are where it makes a difference for me and I'm happy to use 70 durometer sorbothane hemispheres. Without any hard evidence I'd say the sorbothane generally improves the bass end and lower mids. Most noticeably the bass starts to pop out and play tunes.
I'm sure that Herbie's footers or other isolators would work just as well. I doubt whether it matters too much to get the resonant frequency down as far as possible. It could even be
Many loudspeakers start to roll off around 40Hz and almost any form of isolation will get you well below that. Even a few strips of rubber will do that.
pdreher,
I wonder whether the Herbie's gliders under your stands would have worked better as the carpet settled underneath.
In normal use the bass speaker cone should not be able to overcome inertia and cause the speakers to rock significantly, but maybe in your case on top of very wobbly carpet they did, just enough to lose a touch of focus and transparency as in your case.
I guess it might a case of balancing maximum isolation against compromising stability. Too much isolation (excessive wobble) must be counter productive.