Robbo's right. You could use the doorstop/superballs for heavy speakers but you'd need more than four to spread the load and stop them breaking up.

I find that results vary using the system with speakers as opposed to equipment. It depends on the speakers and the floor. At the moment I'm using spikes with my full-range floorstanders and doorstop/superballs with my downfiring bass speakers.

With heavy speakers on heavy paving slabs, try using just doorstops under the slabs. The advantage of this is that any Homebase type doorstops can be used. I have had trouble  sourcing doorstops that are suitable for superballs. I'm currently using Belgium doorstops that I buy in Germany.

On a superball discussion that I was involved in on a US forum, we ended up discussing which superballs were most effective. The translucent ones were considered best and, it was claimed, they bounce for longer than the plain or splurghy-coloured ones. My own tests didn't show this. The translucent superballs look cool though.