Hi, Mav52. Herbie's Audio Lab does not use rubber with any of our products. Rubbers have resonance and reverberant issues that are detrimental to loudspeaker performance, usually affecting linearity. Rubber's contravibrational enerergies also are too slow to deal effectively with much of the very high-frequency acute microvibration that causes a lot of the glare and grunge in music.
We have several dBNeutralizer-based products appropriate to support your speakers. I suggest considering four Giant Fat Gliders under each stand in place of the rubber feet (not under the rubber feet). dBNeutralizer, a loosely crosslinked silicone-based formulation with barium sulfite and silica fillers, is a compliant yet very firm material formulated specifically for the absorption and blocking of acoustical energies. Gliders can be used on bare or carpeted floors, bringing out more of the best sonic potential of the speakers and allowing easier mobility of the speakers when needed. With Gliders, you should get a deeply linear, better-defined bass along with subtle improvements throughout the audio spectrum.Giant Fat Glider
(If you already have spikes to use, a Cone/Spike Decoupling Glider under each spike is an excellent alternative, or Threaded Stud Gliders threaded into the bottom of the stand in place of spikes. With a superior isolation/decoupling approach, there's really no need for spikes.)
Between each speaker and stand, I recommend using four Herbie's Audio Lab Square Fat Dots. Isolating the speaker/stand interface is always most essential to bring out the best potential of the loudspeakers.
Herbie's Audio Lab