Are the speakers on carpet or flooring? One option is to decouple the speaker from the floor by placing the spikes into isolation bases.
See Herbie's...
http://herbiesaudiolab.net/base.htm
or Decoupling Glider...
http://herbiesaudiolab.net/spkrfeet.htm
See Herbie's...
http://herbiesaudiolab.net/base.htm
or Decoupling Glider...
http://herbiesaudiolab.net/spkrfeet.htm