"What is the best surface to place my speakers on?"
  All materials exhibit different vibration/resonance characteristics.
  Metal surfaces resonate and ring easily. Additionally, there is the possibility of creating electromagnetic interference.

  Despite the type and density of a wooden surface, it has a natural tendency to resonate and transfer this into any 
  equipment making contact with it.

  Materials that exhibit high density and hardness such as concrete, granite and some marble (Belgian Black and certain
  Alaskan types) have almost no vibration absorption ability and will tend to transfer vibrations into the equipment from below.

  Glass surfaces respond in a similar manner to other hard materials but with its own characteristic resonance signature.

  In summary, it should be apparent that one needs to decouple both speakers and equipment regardless of the surface that
  they are placed upon.