I also have a basement floor with carpet and padding. Over a 6 mo. to 1 yr. period I tried various combinations of spikes, wood plinths, rubber or composite footers, and nothing. This was for my VSA VR-4jrs. The worst was spikes directly through the carpet into the concrete. Grey, harsh sound with severely attenuated bass. What I finally settled on was a pair of custom maple platforms, 1" thick and about 2" extending beyond the speakers' plinths. Speakers spiked into the platforms, platforms spiked into the concrete thru the carpet. Got back my mid-bass, balanced out the rest of the spectrum, lost the harshness on top. I went with 4 feet on both sets of spikes but I have no doubt 3 would be better; also a 2" thick maple platform would probably be superior too. The platforms elevated my speakers a bit but luckily this did not cause problems re tweeter height and so forth.
