This sound improvement could be due to the composite form of particle board. If you use particle board, make sure to use a carbide blade when cutting, otherwise it will be useless after just a few cuts. Plywood is not a good material because it is full of holes, and if used, the box will vibrate like crazy. Even good quality plywood that I've used had voids in it, so in general, stay away from it. Solid wood looks real beautiful, but it is not a good material because of it's uniformity.
Solid wood can also crack after just a few years.
One of the best box designs is to use double walled, sand filled enclosure. The advantage to this is that the sand will absorb and dissipate nearly all sound, providing nearly perfect transients. The front baffle in the picture is not sand filled because it is hard to not have sand spill out of the driver cutouts, otherwise, all other panels can be sand filled. The absolutely best box design would probably be poured concrete, but due to aesthetic looks, and since I don't want it falling through the floor to the basement, I haven't experimented with that design. It would probably be pretty cheap, though, and there would be no panel vibration.