I have had bad results with vibrapods, fine results with blutack, interesting/inconclusive results with books (just to increase height)...
I haven't experimented with many other [hard] footers under the DCs, but I have with cdps and amps, and there has been a significant difference. I would expect the same to be true of the speakers, probably more so.
I am going to be trying some stands from Skylan in Calgary that use hardened magnets as couplers...