Amplifier Enclosure Resonance

 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.

Plexiglass as a top for a tube component? How could that be better than steel or even aluminum or copper for better heat dissipation or EMI/RFI isolation, aside from rigidity?

Lostbears, I have the same experience with my REF5se. I have tried the Harmonic Resolutions dampening plates and found those to prove a more noticeable improvement over the lexan cover. In answer to question above on the amp, you may try these as well. They come in different sizes, I have the small and am planning to sell those and buy a larger size.