Mystere tube cage

No designer is trying to get the tubes hotter. 

it is a combo power amp and popcorn popper!!!

just be sure to use canola or light olive oil... butter can affect the sound, roll off the treble

Sounds like junk ill pass.

Its for protecting the unit while not in use, like a turntable cover…

It is possible that the design is optimized for cooling, and that the slot pattern promotes airflow. Perhaps a resident *thermodynamicist can chime in?

* A real one, not one of our resident "know-it-alls".

Well, since it is common knowledge, and my experience, that equipment turned on for longer time sounds better, I am pretty sure that the temperature is behind it. Hence my train of thought, however flawed may be, that increasing the temperature slightly more might be beneficial.  It is hard for me to believe that engineers at Durob Audio, quite reputable outfit, would somehow missed the consequences of their design. Especially that their Primaluna line has so different approach. 
Just a food for thought.
P.S.
jjss49 and ebm - thanks for very insightful responses.

I’m sure Upscale will answer your question. Give them a call!
https://upscaleaudio.com/