Daisiu,
When play my 2 or multi channel system I notice that my Stewart Firehawk projector screen moves ever so slightly but it does move.
When I listen to 2 channel music I hear the center image shift from time to time. Some focus on the instruments especially in the midbase loose some of their focus and can seem to get skewed or loose tightness. When I use a bamboo role type shade I do not have the problem. Every thing seems to have more solidity and focus to it and my midbass it more taunt.
With milti channel movies I cannot notice a difference because I have a center channel I think. If I listen to SACD or DVDA I still notice the sound is better with the shade down.
I am trying to find a better and more attractive solution than my bamboo shade but have not found it yet. I don't like thick drapes by the way, thats what most people seem to use. At one time I had hung a rug in the center of the screen. That also did a good jub but the bamboo shade has been better.