Again, I have that exact setup with my Gavia and you can definitely hear any rapping on the granite, sandbox, stand, and in one recent case, on the concrete floor. I've now added Stillpoints between my gavia and the granite shelf. I think there is more to this than just the granite on sand. I believe the plinths on Teres and Galibier handle resonances a little differently so that could be contributing to the differences in our results.
Many Galibier owners use aluminum, 3/4-1", as a shelf on a bed of sand with great results. Makes sense since this is what Thom Makris recommends. He also finds that a similar thickness of MDF on sand gets most of the way to the sound with the aluminum. (Me, I went with the granite because of looks alone, so I'm left to compensate with Stillpoints.) It seems to me that experimentation is the only way to know what works. However, I don't know anyone who has a stand that they can rap on an not hear something through the speakers. I really don't think this is an issue because I've heard my turntable with a arm that had a significantly lower noise floor than my Triplanar and this did not reveal any flaws in the playback. Just the opposite in fact. However, on the other hand I would not claim that things are optimal.
Sometimes people get lucky and quickly find something that sounds good to them. I thought I was one of those, but prolonged listening convinced myself otherwise. I will probably build or acquire a different 'table stand at some point to investigate other approaches.
