Amplifier Enclosure Resonance

I'd call ARC.  They do extensive testing and listening to every unit and every product.  If it mattered I'm sure they'd have addressed it.

They even use Litz wiring tho it makes no sense to me...

BTW, I was surprised at the same thing on the top for my LS25 MkII preamp.  it does have a ton of screws holding it tight tho.

 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.