Hyfi's comment about a vibration transfer to glass shelf is interesting.

Some friends just moved into in a small home and the only way they could set up their home theater was in a very nice custome built-in wall cabinet. which the previous owner had installed. They placed the front left right and center speakers behind grill cloth doors framing the opening for the plasma. Rears are mounted a couple of feet below the ceiling on the rear wall. The limits, shape and size of the room and design of the cabinet forces the subwoofer into the lower protion of the cabinet behind a grill cloth door. Obviously the subwoofer sets off cabinet vibrations during low freq effects. Could putting speakers and the sub on glass pannels solve the vibration problem?

I have seen adds and articals on custom intalltions, where cabinets were built to house all the front speakers including the subwoofer with some type of insulation placed around the subwoofer. Would the use of sorbathane or other insolation materials in the subwoofer compartment help reduce vibration transfer to the cabinet. I have a feeling this won't work because the vibrations are probably casued by the sound waves and/air moved by subwoofer. Unfortunately the obvious and easy fix would be to set the sub somewere else in the room (there is no place) or buy a new smaller subwoofer and hide it in the room (there is no place or money for this). The alternative would appear to be to track down every cabinet vibration and tweek 'em until they all go a way.

Any advise for my friend.