Wayne, It's funny you mentioned filling the enclosure completely, I'm slowly coming to that conclusion.

One of my side surrounds is in an exterior wall that is tightly filled with insulation and the other is in an empty interior wall. I cut out a pocket in the insulation the size of the speaker in the exterior wall I did put a "plug" of insulation above and below the speaker in the hollow interior wall (but I pushed the insulation up/down to create a cavity about 18" high). There is a noticable difference in the sound. I think some is due to the encosure volume and some is due to the sheetrock vibrating. The insulation seems to add support to the sheetrock and stiffen the enclosure, but I'm sure the smaller volume helps tighten the speaker's responce. I do notice that the side speaker with a larger enclosure can play lower bass, but it seems more loose. They have 8" woofers so they still play low enough, even with the minimal/no enclosure volume.

The funny thing about all of this is that when I spoke with B&W tech. support they said the speakers wanted to have an enclosure volume of about 2'500 cubic inches, but then went on to suggest "stuffing some insulation in there...".