If you have a rigid box speaker it becomes less of an issue but I find blu-tak and rubber pads and thick felt (like for furniture) work very well. If you need to resort to ply and rubber layers then it is probably worth considering a speaker upgrade to something that does not have so much cabinet waffle. (you can usually tell a good speaker by tapping and trying to lift it - if you can't lift it then that is often a good sign - if you can cary it with one hand then you will likely have a problem)
Speaker shock absorbers
From time to time people have posted different methods for mechanically decoupling their speakers from the floor to reduce vibration. Some of these have involved using a ply system of rubber strips and wood strips, which seems reasonable to me. I have looked for suitable sized sections of rubber or even for large sheets to cut up but have been unsuccessful. To those who have used that method -where did you find the material and what thickness of rubber and wood did you use for each layer - finally how many layers did you end up using?