I have played around alot with speaker stand fills. My favorite is 50:50 mix of dry sand and steel balls for abrasive blasting (~10 for 5 pounds on ebay). Best balance of preserving low level microdynamics with holding down colorations. I tried lead shot, sand, cotton batting, and combinations of them all.
I also worked alot of coupling and decoupling materials and devices. The best one all around was decoupling with Vibrapods. That one achieved the great combination of warmth of tone along with great clarity.
Of course, decoupling means it now shouldn't matter what I run for a fill any more.
