loss of bass after filling stands

I assume you put the stands back in the exact same spot they were in before filling them. Moving a speaker even an inch can sometimes have a dramatic impact on the sound. Barring that, I suspect what's happened is that you are now hearing tighter, less boomy bass since filling the stands. Tighter bass tends to sound less pronounced. I experienced the same thing when I spiked my speakers. I think you'll come to appreciate the improved detail and tightness as your ear adjusts to the higher quality bass.

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.

I agree with Metaphysics, I think your over dampened. The spikes to concrete should help this and overall focus. You may end up with too much high frequency energy, but my experience with 805's is with bass problems not treble.

Why didn't the B&W dealer recommend the B&W stands that were made for these speakers? My 805's sound tight and relatively deep with the matching stands. The coupling of the speaker to the stand with screws may be a part of it. Also, you may want to try a combination of shot and sand, it may be over dampened as JD and Metaphysics suggest. Macm makes a good point as well. Let us know how it all turns out. Good luck.

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.