@dogberry your system set up for your turntable is fine, enjoy your music and be grateful. Out of curiosity is there anything inside your 400lb 17th-century oak chest? If not perhaps some type of fill might be beneficial, most likely a trial-and-error experiment.
Concrete Slabs
Here I go, perhaps stirring up some controversy.
I have two turntables, both sitting on a 400lb 17th century oak chest. The chest in question sits on a suspended wood floor in a 1985 post and beam house. I just started to play Mahler's 9th (DG/Guilini/CSO) on an SME 10 with an Ortofon Cadenza Bronze. It sounds as it should. But the point is that if I stamp as hard as I can right next to the chest, there is no interruption of the sound. Even if I take a deep breath and jump with both feet off the floor—nothing, nothing at all. So, tell me, what may I gain by pouring concrete here, there, and everywhere (as I believe someone once sang)?
Is this reverence for the ultimate solidity of a foundation the same kind of daftness as when someone says an interconnect must be as thick as their wrist, even though the component may pass the same delicate signal through a PCB trace of minuscule cross-sectional area? What are we aiming for?