Dust cover up or down?

soko, I don't agree with your interpretation.  There was a time in the history of audio per se as a hobby, let's say since the mid-60s,  when the dust cover was an expected part of a turntable, and many manufacturers provided a hinged attachment for convenience.  I never took that as an indication that the manufacturer was recommending anything.  It was just a feature that might seduce a buyer.  In many cases, where there is a hinged DC, the hinges are designed such that the cover can be totally disengaged from them to allow for their complete removal.  I wouldn't over-interpret that, either.

Off

Goes without saying. Off!

Down. To prevent dust settling in the record.

If you are in a fairly dust free environment, then, you could leave it up for convenience.

For me in industrial Singapore, I see benefits from keeping it down.

I have a Technics 1200G.  I've always played with the cover down.  For my own satisfaction, I'm going to try with it up and off to see if I can hear a difference.