Mike, I've seen concrete work but I would float a floor if I had the chance. 2X6 or 2X8 with 12 or 16 inch centers would be great for flooring on a concrete slab. You didn't say what your equipment is so it makes it a little bit harder but I've heard analog gear with friends of mine and you don't want to have a lot of bounce in the floor which is why I would keep the centers closer together.
