If you try the big springs, be aware that they bounce all over the place, but in benign LF oscillations. (Of course they settle down after you cue up the record.) The more mass-loading you give the springs, the lower the oscillating frequency. A heavy sandbox on top of the springs is therefore complementary. Since the entire assembly of motor & suspensionless TT plinth are floated together, the geometry between outboard motor and platter remains stable & fixed. This contrasts to a conventionally sprung suspension such as TNT or HRX. With those sprung plinths and outboard motors, the drive geometry and speed keeps changing with transients of vibration generated from earth and forces of stylus drag, motor noise, and platter bearing friction.
The purpose & effect of the big springs is very different from a conventional lightly-sprung TT plinth. What's particularly surprising is that even with the isolation of a wall-mounted shelf or solid cement flooring, the sandbox set-up still benefits from springs. My theory is that the springs are effective not only in decoupling from earth, but also in dissipating as benign mechanical energy, any vibration generated by stylus, motor, and platter bearing, that is not captured in the sand. I have a custom sandbox much deeper than Bright Star, and yet when lightly tapped on the frame, audible vibration passes through sand to stylus. TT-generated vibration that is not absorbed returns to the stylus.
I think where you'll hear improvement most is in treble resolution, purity & smoothness, which seems still to be lacking after your latest round of tweaks.
The purpose & effect of the big springs is very different from a conventional lightly-sprung TT plinth. What's particularly surprising is that even with the isolation of a wall-mounted shelf or solid cement flooring, the sandbox set-up still benefits from springs. My theory is that the springs are effective not only in decoupling from earth, but also in dissipating as benign mechanical energy, any vibration generated by stylus, motor, and platter bearing, that is not captured in the sand. I have a custom sandbox much deeper than Bright Star, and yet when lightly tapped on the frame, audible vibration passes through sand to stylus. TT-generated vibration that is not absorbed returns to the stylus.
I think where you'll hear improvement most is in treble resolution, purity & smoothness, which seems still to be lacking after your latest round of tweaks.