Folkfreak knows of what he speaks. His room is the best constructed I have ever been in, and is proof of the effectiveness of the ASC Wall Damp constrained layer damping product. I rapped on one of the walls in his room, and the sound produced was that of knocking on a thick piece of slate or a brick---a non-resonant click.
I bought some pieces of Wall Damp (it is sold in 4" squares), and put them between the two shelves (I bought extras, to double the thickness of each top shelf to handle the weight of my heavy turntables) I have on top of each of my Solid Steel racks. A cheap and effective upgrade!