I do agree that the isolation platforms from Auralex are worth using under your sub. The only time they may not be effective is if you are placing them a thick cement floor on ground level, in which case the energy passed through the floor will not result in resonance or vibration of the floor itself. Coupling with spikes in this case would probably yield better results. In all other cases, an isolation platform will remove the interaction of the sub with the floor and tighten up the sound.
I was also going to mention Full Compass considering that the price they will charge will be significantly lower, but they will charge for drop shipping from Auralex, so I'm not sure if it would be a better deal in the end. Full Compass is located about fifteen minutes from where I live just past the west side of Madison, and I have purchased all of my Auralex products from them for a price much lower than any internet retailer, including Sweetwater, and I have been able to pick them up from their facility and save on shipping as well. I also purchased a few items from them for taking measurements of my system, and again I got a fantastic deal; much better than any other chain store or internet establishment. As long as you order by phone or in person, they will quote you a price between 10% and 25% lower than what is listed on their site or catalog.
I also will mention that the expertise and service of the consultants is top notch and that alone is worth dealing with them.