I tried marble over carpet with spiked (sand filled but still very light) speaker stands, resting on the marble and it sounded very thin, though clean, to the point that I kept turning up the volume way to high for my amp. I removed the marble from the equation and things sounded much fuller though a bit rougher. My floors are some type of heavy plaster (kind of like Fix-All) and should be somewhat similar to yours. The stands were by Anaconda with a single round support post which is why they were still relatively light after being filled. Now that I have switched to very heavy/massive stands I was thinking of trying the marble again. I have alway found it necessary to play around with the speaker placement after changing most components (including tubes, stands and IC's) in the equation. Nothing drastic usually just a few inches or a few degrees of angle at the most may end up sounding better to me. My speakers by the way are not optimaly placed by any means, but I will take any improvement that I can get.
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.