Almost all speakers have to be decoupled...what that means is that you're using a device between the speakers and the ground, where the inherent vibrations and resonances of the cabinets flow to the ground, through the devices, and to the ground, then spread, and as a result should be unable to reach back into the cabinets...think of a one-way exit of resonances and vibrations.....if you have the speakers on the ground itself, then you're providing the resonances too much of an area to jump back into the cabinets...if you're using certain materials, such as rubbers in your experiment, same thing happens with a "bouncy" effect....obviously the surface itself makes a huge difference...speakers sitting on sub-floors, tiles, and wooden floors NEED TO BE SPIKED !!!! cement floors will give you more leadway for experimentation..
So, the shape and materials and surface areas are the key here....rule of thumb is s small tip and of course, metal for stability....speaker platforms are fine, as are many other applications...for Pete's sake, even Lego blocks work nicely.... just plain old cones will work wonder and are what I prefer...for a fantastic bang for the buck, check out the Dayton Audio cones from Parts Express....use some double-sided carpet tape and glue them under the cabinets...use 3 pcs...1 smack in the middle of the front, 2 in the back.....
In general, there are 3 areas of huge improvements :
1. Bass tightness...likely that you're losing a Hz or two, but will be much, much more natural with less overhang
2. Sound staging...the above adds to the 3 D presentation
3. Micro dynamics...a more natural bass lets in more information in other areas....
Obviously, different speakers, different results, but I'd venture to say that this would help in over 95 % of all set-ups...