Audio Research has done a lot of testing on the effect the top has on the sound. That is why they went from a steel top to a plexi top on their Reference preamps. I have both the plexi and a steel top for my Ref 5se. The plexi top has a problem with slightly bubbling from the 6550 tube. I really can't hear a difference in the sound though. Personally, I run my Ref 5se with no top for better ventilation.
I have some small leather sand bags that are used by artist. I use a pair on the top of my Ref 110 to absorb vibration. Again I really can't hear a difference between the top on and off.