Dust cover up or down?

It obviously depends on your TT and your ears.  

Quite from Hoffman forum:

"Bottom line: If you don't notice much of a difference, then down all the way. But just having the lid so at the minimum you can close it when you're not using it will help a lot."

Fuzz, I don’t get the last clause of your quote. Leaving the cover up and hinged to the rear edge of the plinth would seem to be the worst possible choice. Fora couple of hypothetical reasons.

Off. It literally opens up the sound when it's open or removed. If you don't notice much of a difference, you need better gear.

Bottom line: If you don't notice much of a difference

I don't use it at all. In fact my Avid Ingenium does not even come with one. On my Music Hall MMF-7.3, I even removed the dust cover hinges....

People spend tons of money to reduce any adverse effects on their turntable caused by vibrations. Removing the dust cover while playing a record is probably the cheapest way to reduce microphonic vibrations that will cause distortion in sound. Some cheap turntables with cheap cartridges may not make much difference with or without a dustcover because the sound is already pretty bad, but very revealing high quality cartridges may suffer from such vibration especially when you listen to music loud with the dustcover up. The dust cover will pick up the sound wave, however small, which may well cause vibration.