Since we're getting into horse puckey here, how 'bout an idea Jon Best reminded me of a while back - horse mats. Any decent tack shop or farm supply carries these regurgitated tire material compressed rubber mats, they are typically 4 feet by 6 feet by 1 inch thick. They can be sliced with either a recip saw using a medium tooth blade, or a razor knife and steel straight edge (if you're tough) It would take a little experimentation to get the right size "pucks", like build a small section of floor and load it with varying weights to find optimum compression - you would need to know the weights of all the building materials you're using, and whether you're bearing the walls on top of the floated floor or building them outside the floor (I've seen articles showing both ways)
As far as synthetic vs. springs, different animals here - Kinetics labs sells rolls of hockey pucks on backing, to be rolled out and covered with the layers of floor material, so neoprene apparently is OK for that.
Yeah, for a guitar amp a tire should work cheaper than those new isolators from Auralex (I think) Maybe not as classy, but if you get one of those hop pants/Ferrari posters, who'd be looking? Or, you could do like the Beatles did for the one album, and everybody just wear shoes with Rubber Souls... Steve