I wouldn't put anything under the speakers. These need to be as stable as possible so that the driver's movement are "true" and not the combined result of the whole speaker rocking back and forth on a foam pad. It's a bit far fetched, but if you're that concerned about the sound, then I believe that's what you should do.
Isolating the receiver's feet might be a good idea also. But I doubt it'll make much of a different, it being completely solid state.
Now the CD/DVD player is an other story. That guy should be minimally isolated. Still, the inner suspension of the driver should be able to cope with most vibrations.