Subwoofer Isolation Pads/Feet
Before spending $50(or more) on these devices try using the inner boxing foam that came with your subwoofer temporarily.
The shipping foam is usually similar to the material used in the "isolation pads". Place the foam under each corner of the subwoofer raising it up an inch or two. Then audition your favorite movie and music demos. If you hear an improvement you feel is worth the cost...go ahead and order the actual isolation pad. Or, save your money and just keep the inner boxing foam under the sub!
Either way, don't talk yourself into hearing magical improvements here. If there were significant improvements they would be *well* documented by now. You may change the way the subwoofer interacts with the room environment slightly, after all, you are moving the subwoofer. And I suppose the argument could be made for a *very* minor reduction in the transfer of energy directly from the subwoofer enclosure to a hard surface/flooring. But if the bass suddenly seems much more "accurate", less "boomy" and full of wonderful amounts of "bloom" this could be due to the psychological bias that will be inherently tied to such a sighted listening test. Don't beat yourself up about this though...we all have these biases. It is simply a part of human nature.
This presentation delves much deeper into the scope of my thoughts here if anyone is an AES member
Power Sound Audio