What do you put underneath floorstanding speakers?

For my B&W CDM 9NTs, I recently put a 1 1/2" thick soft wood plank(14"x 10"x1 1/2")not MDF, with amazing results.IMHO the improvement is dramatic with the CDM series because the spike quality is awful.You can effectively embbed the speaker in the softwood such that there is no lateral movement.Of course this home spun technology will only work for relatively lightweight speakers. The 9NTs weigh 26kgs or so, or 60 lbs.
Have a great new year.
Sunnyboy