I believe it depends on how solid your turntable is in correlation to how much it will affect the sound. When you have the cover up it has to gather more sound waves like a sail on a boat. The best would be the kind that does not touch the turntable. If you notice really nice turntables do not have covers attached to the table itself. Cover off while playing is the best. If you want to test it. Have some one listen to the speakers while you talk into the cover with the player on. I keep my record player in a cabinet sitting on a separate plinth with dampers so the sound from the speaker does not affect the music. Like someone said about a separate room. I just isolated it from the affects of the room.
Ok guys
My question is this, nothing major, more of a curiosity. When playing an album does it make a difference if you leave the dust cover up or down? Probably stupid question but hey there are lots of opinions out there