I would fill them if the stands you have allow it. Not only will it increase weight and stability, it will reduce resonance.
Here is another alternative to sand. I use crushed industrial quartz in my 24" monitor stands. Cost about $3.00 for a 40 lbs bag at your local Honme Depot, Lowes, etc. Its cheap and heavy but does give off dust while pouring. Just use a funnel while you are filling your pillars and when full, simply wipe away the dust that settles on the stand. So its best to do it outside or in a garage. I have a pair of B&W STAV24 stands and I filled each pillar to the top and each stand took about 20 lbs of fill. So a single 40 lbs bag was ideal. They are very, very stable.
Here is another alternative to sand. I use crushed industrial quartz in my 24" monitor stands. Cost about $3.00 for a 40 lbs bag at your local Honme Depot, Lowes, etc. Its cheap and heavy but does give off dust while pouring. Just use a funnel while you are filling your pillars and when full, simply wipe away the dust that settles on the stand. So its best to do it outside or in a garage. I have a pair of B&W STAV24 stands and I filled each pillar to the top and each stand took about 20 lbs of fill. So a single 40 lbs bag was ideal. They are very, very stable.