Ok guys

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!

stani? When you put on hearing protectors does your head rumble and your tongue vibrate?
Lewm, tell me your system is in a clean room. That would be a $250,000 upgrade. Records that have dust ground into them do not sound so hot.
A dust cover has to be hinged to be useful. Again the best way to do this for unsuspended tables is to have what I call a platform dust cover. You have a lexan plate on which your turntable sits. The dust cover is then hinged to the platform. If there is limited space behind the turntable hinge the dust cover at the top with the back fixed to the plate. This version requires a prop to keep it open. If you have plenty of room behind the table the you can hinge it at the bottom and the dust cover will counter balance itself so friction hinges will do the job. You can use any thickness of lexan you want. I would bet that 1/4" lexan would drop sound 30 dB under the dust cover. That would be like putting your turntable in another room. These guys will do anything you want,  https://www.displaycasej.com/custom-audio-covers. 

I was playing with my dust cover up and down last night when it dawned on me. There can be a significant delay in the sound getting to the turntable depending on where it is in the room. If the delay is long enough this causes an echo which can give you the sense you are in a larger room which might appeal to some people. A euphoric distortion. I would rather listen to the acoustics on the record. 
Well for once VPI and I are on the same page. Go figure.

I would not have a TT without a dust cover when not in use. It's a great place to place a tray of house plants on, while keeping everything clean, especially the refined moving parts of the tonearm.

Open or off when playing. If you believe in microphonics, open, let em go.

Hinged dust covers up while playing? Keep a bubble level on the deck. Does it change when the dust cover is on-up? surprisingly a few I had did not effect level. 

If effects level, you can remove the hinges and just put it on whenever.

Seriously, we bought a very nice unit for audio equipment and I told Donna she could have the top glass shelf , table height, for plants. Well, eventually, to get some stuff on that shelf, and keep my word, I painted 2 cafeteria trays, big felt strips on the bottom, and they actually look better higher up. I trust her watering skills. Just move the tray off the TT to the top of the Reel to Reel Dust cover, play on.

Mijo
It would be nice if for once you could just accept the fact that your opinions are going to differ from other members.
Nobody is arguing that what you state in your system is what you hear and great for you.
However maybe you could just accept that it is in fact just your opinion and not try to foist that off to everybody else here as being gospel.
It works for you and that's wonderful, apparently it does not work for a lot of other members myself included and that should also be wonderful, not an area of contention.