One obvious answer is to remove the spikes altogether. I've yet to hear a speaker that sounded worse without spikes.
Failing that, another vote for Herbie's Decoupling Gliders,
http://herbiesaudiolab.net/spkrfeet.htm#cone
Failing that, another vote for Herbie's Decoupling Gliders,
http://herbiesaudiolab.net/spkrfeet.htm#cone