Viridian
Given that any structure has a resonant signature, the question then becomes not, "which is the most dead" but "at what frequencies are the resonances in the structure and what is the amplitude, magnatude, and Q of those resonances?"
+1 Excellent Viridian
One example of this.
"Granito is a material composed by little pieces of marble of very different origin agglomerated inside a mold with cement. Machined and polished. The resonance of the plinth with its suspension is about 5Hz and it is well absorbed by the air cavities"
Mr. J.C. Verdier (R.I.P) in describing the Verdier Vintage La Platine Granito.
Along with La Vintage Platine, I also "still" own two full plinth TT's, My 100 lb Jean Nantais Lenco, and an SP10MKII that can actually be put in one of two configurations. 1) A Full Plinth - 70 lb Multi 6 Layer Birch/ One layer MDF or 2) a Rigid (in your words ParrotBee) Skeletal Plinth.(both plinths which are DIY sourced and put together). The SP10mkII is sort of a pet project going way back now. fwiw I prefer it in its Rigid Skeletal Plinth form. It is more neutral sounding.