Are Dust Covers a MUST

Concur with Vtvu. Having a dust cover is probably going to make the system sound worse. ime, I tried using Cardas RCA covers and they noticeably imparted a metallic signature to cymbals that was not there when they were removed. The only issue was that they decreased the noise floor significantly so I went looking for replacements. I've recently picked up the Acoustic Revive units which I hope won't have the same drawbacks but will have the noise floor benefits.

Yeah Dan but it was a toss up between that and the Cone of Silence in the chief of Control's office from Get Smart.

LOL!!

Which dustcover would be better if it has to be used?
One sits on the plinth or one sits on the rack that the turntable sits on? If the latter is better, why there is not many companies that make it?

After much experimentation (this afternoon) I think I like it better OFF. It seems to have a bit more air.

Whether it is all in mind or not is...the eternal question.