Just tried this on the XBA 40s. That "muddy" sound does go away. There is more detail to every instrument while keeping every note "full". It does make them sound a bit more analytically as you does sound like there is a boost in treble, but the different is more due to removing a filter, not because you did some equalizer shift like some of the people on here are trying to make it sound like. Completely different story. I do notice the bass has less of an impact, but it "fixes" these headphones. There was so much "bleed" into the mid-range and weak detail. This sure improves upon that. You can throw the foam back in if you want to afterwards. It is reversible.
Do note the foams are glued in.