I did a .357 caliber upgrade to my stands for use on a hard wood floor. (Get rid of those skinny needle type screw-in spikes - whatever you do.)
SeriousIy I epoxied .357 caliber bullets onto the bottom of my metal stands. Good ol' lead has long been an audiophile favorite - its very dead - no ringing. And very soft - will not likely mark your floor at all. I used something called a semi-wadcutter, which is ammo used for target shooting - it has a flat end instead of round or pointy - tapers just a little.
BTW - I used just the bullet/lead portion, not the entire cartidge with the brass and gunpowder. Bullets (as opposed to complete cartridges) are sold for reloading ammo. I supposed I could sell a handful for a couple bucks plus shipping - otherwise you gotta buy several hundred at a time - but they could be used for balast in stands or anywhere else you need a little lead.