For an explanation go to: http://www.stereotimes.com/
the "Archives" section and the article "The Art and Science of Audio System Tuning, Part 4."
I've experimented with their recommendations, and found that wood blocks make changes in your system and those changes vary with the type of wood used. I've even measured the differences with my SPL meter using a test record and different frequencies yield higher or lower Db levels depanding on the type of wood. But you don;t need a meeter because the differences are clearly audible. I found zebrawood to be my favorite and is now under all of my equipment - I wouldn't be without them. I did find an even greater improvement with something like a Vibrapod placed between the wood block and the equipment shelf - equipment on top of wood block on top of soft absorber on top of shelf. V-Pads - rubber/cork absorbers - make a good cushion under the wood block (or under anything, for that matter)- V-pads are cheap @$2. and available heating/airconditioning suppliers.
But you don't need to spend a lot to experiment with wood. Simply go to a local hardwood retailer and buy 1-2' of a few types of wood, cut them into 1"x1"x2" or larger blocks; at lease one dimension should be longer than the length of your equipment feet so that the block can be placed directly in contact with the equipment chassis completely avoiding the equipment feet. And, it seems best not to set the equipment in end grain.
Give it a try. You'll be amazed what changes such a simple and cheap tweak can bring. Depending on your system and the type of wood you use the change can be very positive. Have fun!
p.s. If you can't find any hardwood, let me know and I'll send you some of my extra blocks at no cost.
