I'm experimenting with tennis balls. My listening room is above a two-car garage so the floor is wood.
First, I had the subs sitting on the carpet. Next, I tried them with spikes on. Finally, I tried the tennis balls. There are 15 under each sub.
Because all three drivers face forward, the weight of the magnets make the center of gravity more towards the back. I placed the middle row of 5 balls closer to the back.
I was surprised at how little the subs wobble when you push on them. They do not move around easily at all. My wife says that the bass is not as loud in the rest of the house now.
I am happy with the performance that I get from these, so now I will make a black cloth skirt to hide the bright yellow balls even though my wife likes the look of them the way they are.