My stand is well damped with glass, metal and sarbothane in between.Glass and (most) metal are not damping materials.
Glass and metal typically reflect higher frequency energies at the boundary layer. Imagine a room consisting of a big steel box with large glass windows. Stand inside and play your violin loudly. Would those windows, walls, floor or ceiling "damp" the sound? I think not.
Lower frequency energies which do cross the boundary layer into a glass or metal layer tend to be propagated rather than damped. Closing a window attenuates birdsongs because their high frequencies are reflected, but the low frequency growls of the garbage truck pass through the glass and are still easily audible.
Some materials do damp vibrations. The dense hardwood of your Reed tonearm is one such and that contributes to its quietness. But glass and metal typically do not.
By design, phono cartridges detect and amplify vibrations. Therefore, any energies reflected or propagated toward them will be interpreted as signal, which raises the background level of sonic mud. Including sonically reflective or propagative materials in either the TT stand or the plinth raises the system's noise floor.
Of course you should buy whatever plinth you like for whatever reasons you wish. Just thought it might be helpful to clarify some mistaken assumptions as regards sonics.