Efficacy of a sandbox depends on the turntable design in my experience. With my Acoustic Signature Final Tool turntable a heavy sandbox made no discernible difference. However, replacing the factory "cone" feet with Stillpoints did make a difference. I also imagine what the sandbox sits on also is a factor.
In the sand box
I have 2 audio buddies who have built component sandboxes with different types of sand. They claim Beach sand is the most dense and made a huge difference when the turntable and amp were set in them. Has anyone else been down this road? I mean there are several ways to isolate equipment, this is just one of many.