I bit the bullet and reduced the enclosure volume on my front L & R and center channel. They were previously about 5'000 to 6'000 cubic inches and I added an interior partition to bring the volume down to 2'000 cubic inches. I put fiberglass insulation (compressed to about double it's normal density) in half of the remaining enclosure volume. The bass is much tighter, the highs continue to be strong and the mid-tones are coming through stronger.

It is an improvement over the huge enclosure volume with lots of insulation and foam, but I am getting diminishing returns. I tested the Austin Powers Gold Member DVD and the sound was very good, especially considering my huge, odd shaped room. Music is also improved though not to true audiophile levels.