Filling speaker stands: sand, lead, steel, rice. Full, half, etc?

With my proac tablettes, I am using 28 inch speaker stands which have two round steel tubes about two inches in diameter.  The stands have spikes into my wooden floor (through carpet) and rubber pads on top.  I have a rather small room 12x12, and I have played around with the speaker positions in the room, finding a pretty sweet spot optimizing the bass and soundstage.  I have done all this with the stands unfilled.

so I am looking for advice on filling the stands. What do I use: sand, steel shot, lead shot, rice?  Do I fill them half full, three quarters, or full? Do I fill all the tubes the same? Can overfill makes things worse?

like most things (i.e. speaker placement) I suspect there will be some trial and error, but as it will be a real pain in the rear to fill/empty/refill the various materials to really compare, I am hoping that there are some lessons learned that the rest of you can share with me to get me pretty close with my first trial.

Alos, what effect will the filling have- should I expect deeper bass, bigger soundstage, greater depth, more detail, or..... better yet what should I not expect to change? This will help me know what I should be listening for as I tweak things.

and finally, should I expect my current optimum speaker position to change with filled stands? Will I have to go through the whole positioning exercise again?

thanks, ( and happy new year)

Bill