Spikes versus Rubber on wood floor?

Chadnliz -Thanks for the encouragement- its an expensive experiment for me but finally getting to listen to great speakers with great equipment ignited a lust for improvement and you have to start somewhere.
Thanks everyone for the suggestions which fairly clearly eliminate rubber products. The joist supports on 12 inch centers were designed to have a large grand piano placed anywhere with no support underneath for a 30 foot span, so the floor feels completely rigid to the jumping test. I had not heard of the black dots - looks to be an intriguing material.
I have some cermaic tile pieces about the size of the speaker bottom, may compare the stock spikes on these to the balck dots.

Gammajo, here's the deal on speaker placement over joists:

Assuming the joists are running the short way across the room (and that you have your speakers facing the long way) take care to place the front and rear spikes of the speaker(s) as close to adjoining joists as possible. In other words, if the front and rear spikes were actually 12" apart, then the front spikes should be right on top of one joist and the rear spike on top of the next joist to the rear.

Or another example, if the F and R spikes are 16" apart, then the fronts should be 2" in front of one joist and the rear 2" behind the next joist to the rear.

You can locate the joists pretty easily with an electronic stud finder if you can't actually get under the floor and see them.

That is what I meant by "cunning" speaker placement relative to the joists.

Nsgarch - Got it, if the speakers do not rock back and forth over a single joist, they will rock better on the music.

Exactly!

If you want the speakers to stand as still as possible while playing, spikes are the least likely item to do the
job! Quite the contrary to many peoples belief, they act as a "tuning fork" and excite floor resonances. These floor resonances will be perceived as delayed sound, that is in no way in harmony with the origional sound.
Decouple the speakers from the floor ,and you are more likely to obtain a good result. Don´t just use rubber feet,instead try some "soft" feet that are made from a material that is in accordance with the weight of your speakers. You have to weigh your speakers at their front and back to order the right stuff.Here in Sweden , there is a brand called "SD-foten" , but there are more brands than this one. The resonance frequency of the decoupled speakers should be in the vicinity of 8 Hz , I think.

An Italian reviewer thought he heard doppler-distortion, when using such devices as SD-foten , (because he was able to rock the speaker back and forth). Talk of expectaitions ! Measurments has shown the opposite to be true.Even a Swedish hi-fi magazine that was sceptical at first , did find it possible to place a coin on its "edge", play loud while the coin was still standing. If plain physics would role , instead of marketing "hype" , good sound would be more common.