I was doing some upgrades to a set of Monitor 11's the other day and discovered something that may be of interest to the owners of said speakers. Inside the cabinet, under one of the mid-drivers is a 3" thick sheet of Sonex Classic polyurethane foam. It is glued to the cabinet side and to the brace in the middle. In one of the cabinets I was working on, the Sonex material had fallen down. As I pulled it out of the cabinet, it crumbled in my hand. A quick check of the material in the other cabinet showed the same results indicating that this stuff has reached the end of its life span. Therefore, you will want to check your speakers for this issue.
The solution seemed easy enough, buy and install some new Sonex. But NO! I could not find anyone willing to sell a single sheet of Sonex Classic, you have to buy it by the case at about $250.00 per. Eff that! After looking and looking I was able to find one company that had some Sonex melamine foam in 2 1/2" thick sheets that was willing to sell me a few sheets at a much more reasonable cost, still not cheap though. I may find a use for the extra sheets, but maybe not, so if you need some drop me a line.