Arthur,rubber will not really lead to the disposal of any vibrations from the cd player to the rack,but may of course lead to dampening/absorbsion of any vibration from the cd player.The rubber,will protect from vibration from the rack reaching the component.I guess this also depends on how hard the compound of the rubber is.I used squash balls cut in half under the feet of an hybrid amp ,at one stage.It did make a difference.
Funny,a while ago ,I thought this whole "vibration" control issues were nonsense, until I tried it myself ...