DIY Stereo Rack

I want to build a music system rack but get a headache after trying to chose what route to go. Is the hardest densest wood like ash or walnut best. Or something softer? I was thinking of using countertop cement for shelves. My thinking being the harder it is the easier vibrations transfer. Cement being not so dense may not transfer vibrations so readily. If styrofoam was stronger why not use blocks of it for super vibration isolation? Thanks for any insight you can provide. Bob