I've recommended the Diversitech product for isolation on many occasions. Good call!
I have heard the rubber can mark/discolor wood: I cannot verify, though.
Also... somebody once asked about using that as a base for a spike. No go. Those guys are only rated up to 50PSI for them to be effective. Pretty sure a speaker spike will far excede that rating.

I don't think I would want one of those spikes pressing against me with only 20# applied to the other end!