If you really want to INSURE the safety of your monitors, you have a few options.
1) Drill holes into the bottom of the cabinent and secure it with bolts.
2) Use some type of tiedown that goes around the top of the cabinent.
3) GLUE the bottom to the stand. I don't recommend it for obvious reasons, but it works.
In any case, the stand is only as stable as the amount of weight applied to it. Having a large footprint for the base reduces the amount of leverage that force applied to the top of the stand will make. Also, sand is very good, you can also add lead into the sand for more mass. This is mainly acoustic, as the stand itself will still not be heavy enough to counter body weight (of all things). The PVC is resonant, but sand is very good at damping, so no worries about sympathetic vibration coloring the sound.