I've been thinking about your spring arrangement for some time, I'm willing to give it a try. Should be good experiment with my wall shelf and huge sandbox. The only issue is determining which springs to try, I will have somewhere around 300+ lbs to support.
While I can understand the negligible effect of the springs on the movement of arm and stylus, not reintroducing vibration into tt, and greater de-coupling from external forces. I'm still not sure that internal tt vibrations can travel down springs into the stable surface below that, seems to me vibrations would get caught up in springs, both external and internal generated energy would be fighting each other in spring. I would think those vibrations would have greater drainage through the very sharp spikes I'm currently using.
I suppose the greatest isolation I could muster would be wall shelf supports made from a more aborptive material than my present steel supports. This would both serve to better absorb energy from tt, sandbox vibration sinks, sandbox, maple platform, and de-couple from external energy.