The Monitor Audio Silver 5i's that I previously owned also had a chamber in the bottom of the cabinet. There was a plastic plug covering the opening. I used around 25# of lead shot (double bagged)in each. It tightened up the base considerably. The 25# was probably overkill.
If you do use sand, make sure that it is dry. Bagging it would probably also be good.
Chris
If you do use sand, make sure that it is dry. Bagging it would probably also be good.
Chris