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.