Speakers to perform their best MUST NOT MOVE AT ALL (meaning the tops should not wave around, even a silly millimeter!). So rubber is OUT! Spikes are best, but if you are on a wood floor, you have to be cunning about where you place the speakers relative to the supporting structure, in order to minimize the rocking motion that can result from the floor flexing (spikes won't help with that problem.)
So, how far apart are the trusses?
So, how far apart are the trusses?