Nickbuol gave me some good advice about fabrics on the first page of this thread :
... you can use a basic solid muslin fabric (a bit too "industrial" for me), or really any fabric that you can literally blow through... Try putting the fabric up to your mouth and think, "If I was a hostage, and they put this over my mouth, would I be able to breathe, or would I pass out." If you can breathe through it, then you are pretty good for being passable for covering an absorption panel.
After doing a bit more listening/testing it occurred to me that the deep bass notes sounded just a bit like someone thumping on the bottom of a big Rubbermaid tote ... so maybe I'll try some solid speaker stands before I replace all the furniture with beanbags.
