Ok guys

This discussion illustrates beautifully the problem with technical attributes. The subject at hand is dust covers. But it could just as easily be amplifier watts (is it better to have more or less?), speaker cables (is it better to have silver or copper?), or any of a hundred other technical items. 

Everybody wants a simple answer when the truth is there are no simple answers. Its better off, probably, unless its a table designed to be better with it on and down, in which case it should probably be on and down. Probably. The only thing we know for sure is which way it sounds better determines which way it goes- and not the other way around. So once again might as well just cut out the middle man. Go and listen. You will see.

It's good to have a dust cover to keep your table clean when not in use.  When using, do some blinded tests and see which you prefer.  If you can't tell the difference, then play records without cover so you don't have to bother with it when changing sides.   

...The stylus may "resonate" inside the dust cover, creating unwanted reflections. In addition, if the cover picks up vibrations, it creates an internal rumble that aggravates what has already been said. We must try

By no means do I have a "fancy" turntable, its a Pioneer PL-550.

But.... I've tried listening with the cover down, cover raised, and cover completely off.

The best sound achieved for me was with the cover completely off. My turntable is about 10 Feet to the left of my other components / listening area, and with the cover down or in the raised position, I get some stray vibrations / rumbling. (it is worse with the cover raised).

@kewm  Kenny G?  Kenny G??

Good grief, man!