You know I've thought about this off and on in the past and wanted to come up with a cheap DIY that would test if it would be worth pursuing. I came up with one idea. If you can imagine a bag of sand (or better lead shot) say 12" long 3" wide and about 1" thick - now sew as many of these together (kinda like a bullet belt) as required to drape over your speaker's top and sides. Now the back wouldn't be covered unless you made a belt for it, but this method was only just a test I could easily fabricate and try out and would only be applied during listening sessions. It's not a very aesthetic thing but it could work I think.
