Anything that changes the height of the loudspeaker or that changes the distance from the units to the floor will change the sound. This is because reflections change. Also moving them forwards (always good) or backwards (usually not good) changes the sound and stage.
If you really want to place them on a platform I would go for something as solid as possible, marble comes to mind. Then use cork dampeners inbetween (really no use spending a fortune there), and you should be set.