Do speakers sound better on wood floor or concrete

My KEF Reference Twos are on a tile ledge about 8" high that is part of a built in wall unit. The ledge is the bottom of the wall unit and extends out about 17" in front of the shelves. It is hollow underneath the tile but fairly solid. I'm currently just using the round feet (no spikes) that came with the KEFs. Should I mass load the plinths, add some rubber or sorbothane under the feet or put on the spikes? Anything else I can do to tweak? My components are on the same ledge in between the KEFs but they have 4 Vibrapod Cones under each piece. Should I put Vibrapod Isolators under the round KEF feet?

Thanks for any advice,

Scott