It boils down to cost of the filling material verses the possible difference in sound. Depending on how large the cavity is to be filled, lead can be expensive and I am not sure one can hear the difference between fill material. Some day you may want to empty them to move, sell or ship them.
Lead shot - heavy - expensive - toxic (lead dust when pouring)
Steel shot - not quite as heavy - expensive
BB's - very expensive - heavy
Small pebbles - cheap - somewhat heavy - needs a larger opening
Play Sand - cheapest - easy to pour - might sift if the stands aren't sealed.
Kitty litter - not recomended
Lead shot - heavy - expensive - toxic (lead dust when pouring)
Steel shot - not quite as heavy - expensive
BB's - very expensive - heavy
Small pebbles - cheap - somewhat heavy - needs a larger opening
Play Sand - cheapest - easy to pour - might sift if the stands aren't sealed.
Kitty litter - not recomended