Dear Raul,
Till today IMHO and with all respect to everybody we only have or are: charlatans on the subject.
I don't believe that there IS a "best" TT plinth material.
Just as there are tonearms made of aluminium, titanium, wood, carbon-fibre and plastic all competing at the same level, indicates to me that all materials have some benefits and some disadvantages and it's really the implementation and compromises made, which determine the ultimate worth of a component such as a turntable or arm IMHO?
Just as we debate whether the virtues of DD, belt drive, rim drive or idler drive for turntables will tell us which is BEST, there will never be universal agreement.
So instead of calling all the turntable designers "charlatans"?.....let's be grateful that throughout the last 50 year history of turntable design and manufacture, they have provided us with a practical 'laboratory' of drive theory and materials application from which we can learn an immense amount if only we knew how?
Regards
Henry