dustcover up, down or off


To get the best sound from a TT, should the dustcover be up, down or removed all together? I have always removed my dustcover, but I recently read an opinion that having it in place prevents unwanted vibrations from the speakers from reaching the TT. What are your thoughts? Thanks.
tbromgard
I used to own a Music Hall mmf 5.1, and noticed within the first month of ownership noticed that it sounded much better with the dust cover removed. What everyone else is saying here is correct: it acts like a microphone and causes the plinth and other components to resonate.
I bought an MM7 (used with no cover) and replaced the cover with a new MM7 cover. When the cover is down the counter weight starts to hit the cover during the last 1-2 minutes on the last track of each LP, so I leave it open. Stevecham: does this happen to you?