...I think that the give afforded by even short hard rubber or cork is too much. My thinking is not so much "draining of resonances to ground" as much as making the speaker itself as rigid in space as it is reasonably practical to do and to that end, a little give at the speaker stand base translates to substantially reduced rigidity at the speaker. ...
Russell, sufficiently hard rubber will give virtually no 'give'. If the speakers have reasonable heft to them then any 'give' would already be taken up, compressed by speaker weight ...especially using 3 instead of 4, and thin washers. The resistance of the very hard and (virtually) maximally compressed washers, together with the weight of a decent speaker, should help minimise any rocking.
I don't have an accelerometer or whatever is required to test movement of the speaker.

while writing this I just had a thought ...if you had (preferably identical) bathroom scales, you could put them under the front and back feet of the speaker... supposedly, any rocking motion would appear as rising & falling readouts. Or, you could put the scales under the front feet and a block of wood (to keep speaker level) under the back feet. Unless there is substantial swings in the readout on the scales, there will be insufficient force to further compress the washers and cause any rocking.
It's fine you disagree ...it challenges us to think things through
