sand in speaekers

I would contact the speaker manufacture before doing this, as this might reduce the volumn inside the speaker and affect the sound. Another Option is to put a #7.5 lead Shot in bags atop your speakers (Common for subs). Make sure to double seal the bags and keep out of reach of children because of the lead.

I'm pretty sure he is talking about specific models that are designed to have mass added to them.

I can't think of any off the top of my head, but I know there are manufacturers that make a cavity available for this purpose.

Totem Sttafs can be filled with sand, shot, or other material. There is a cavity at the bottom specifically for this purpose. I think all of the Totem tower speakers may have this provision. When I had my Sttafs, I filled them with 5 lbs of lead shot.

Also. my Von Scweikert VR3's have a fill cavity in the Mid/Tweeter module - but this is up top, not on the bottom.