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.
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.