Hi Topside,
Thinking about it I remembered that in the 1980s I had 2 amps that I removed the top cover in the summer for better cooling and surprisingly the sound improved a bit without the cover, then I would recommend you use the amp without the top cover, unless you live in Canada, so this way you don't will need dampening the cover, and when you are finished listening put the cover on top of the amp without the screws (store them separately) until the next audition.