Floor Resonance and Full Range Speaker Stands

Supporting the floor from below is the best way to go, but if it's on the second floor or some location where that is impossible there is an alternative. Get large slabs of granite. The bigger the better. These will not move much, and will distribute the energy to a large enough area that it will not cause the floor to vibrate as much. You could even try to put some sort of absorber between the granite and the floor (such as a dense piece of carpet), but due to the weight of the granite that might not help much. As to where to get granite--well you can go to the kitchen counter place, but that's going to probably be pretty expensive. It's better to try the guy that makes tombstones and mispelled someone's name one day--that piece of granite will be pretty cheap.

The concrete slabs should work fine--and it sounds like the price is right. Granite is probably a little better because it's a denser material--but I don't think it would be much better. Give the concrete a try--and let us know how it works out.