In addition to filling the stands with sand, I would suggest that you get rid of the marble and put the spikes right throught the carpet and pad to your concrete floor. The marble slab is "floating" on the carpet. Having the spikes go to the concrete will give you the tightest bass and most stable mounting of the speakers. In my experience, the spikes will do no damage to carpeting.
loss of bass after filling stands
Have a pair of B&W matrix 805 and purchased Lovan Ballet stands at the suggestion of a local B&W Dealer. Listened to them for a week without filling them. After filling with sand I sense a loss of some low end. Has anyone experienced this. I also have them on marble tiles as my floor is carpedted(over concrete floor). Any suggestions.