Before you pour the concrete you may want to consider my experiences. Reinforced concrete does have a very noticeable resonance. That is why industrial labs use fairly elaborate vibration control engineering in critical applications and don’t just use poured concrete
When I was doing earthquake retrofit, I poured some large concrete piers in the crawl space to disconnect my Dunlavy SC-V's from the wooden floor in the listening room to control the floor resonance. By moving a simple vibration detector (water in a shallow bowl) over the concrete below I could detect the standing waves caused by the speakers resonating the concrete, primarily through the reinforcing rods. There were definite null and reinforcement points. Your basement floor was probably reinforced with mesh so your resonance might be less, particularly if you spike your speakers.
However your reinforced poured concrete pillar will still have a very definite resonance, which would be a bad thing for a turntable stand. It might not matter since it would be difficult to calculate in advance. In my case the fact that the concrete sub floor vibrated didn’t really matter. I was just curious about the resonances. Once I substituted oak bracing for the metal floor jacks that I started with, the vibrations were significantly damped. BTW, separating the Dunlavy’s from the wooden floor made a BIG difference in the measured and perceived sound.
