Granite Slabs

Amazon sells machine plates at a reasonable cost. If you are lucky and the dimensions fit your speakers footprint, you might be able to score a pair for an acceptable price. Otherwise, going to a local marble dealer would be the next alternative.

I own Vandersteen Treo's. Mr. V. swears by adding a 3 inch granite base allows the speaker to perform optimally.  I own the Townshend speaker podiums, but added them to my components instead of the speakers, where they have clarified the bass significantly.

Bob

I bought travertine tiles from Floor & Decor.

@skeptikal ,

That's a awful lot of brain you're toting around there.

Unrelated question: Where do you buy your hats?

Let’s break it down…:

If you have carpet on a suspended floor, such as on the second level of a house, spikes are useless. Spikes are only good to go through carpet onto e.g., a poured concrete floor. You need to dampen the vibrations.

Mass such as granite slabs does NOT reduce or isolate vibration. It’s a common misconception. Mass stores vibration and transfers it. It doesn’t absorb it. 

So, you need to dampen the vibrations. As some people here have already suggested, the Townshend platforms are great. They will you run you $1000-$1500 per speaker, though. Well worth it, so it’s a budget decision.

Another option is isoAcoustic dampeners. The version you need will depend on the weight of your speakers. And since you have carpet, then you add isoAcoustic  carpet spike plates under the dampeners so they don’t ride directly on the carpet.

Hop on their website and learn more about the dampener - weight options and the carpet spike plates.