Tonearm mount to the plinth vs arm board vs rotating arm board vs isolated tower

I agree with @halcro , it is essential to isolate the tonearm from the platter bearing and the motor.

I also agree with @atmasphere , it is essential to have the tonearm rigidly connected to the platter bearing.

I also agree with @lewm , bolt the tonearm to the plinth if possible, but compromise as necessary.

There is a material which is highly rigid, on the same order as aluminum plate, which is also very highly damped. That material is panzerholz. I built my platter board out of panzerholz and glued a layer of carbon composite to the top for further rigidity and constrained layer damping. Then I bolted my tonearm to the platter board.

Bearing is all air - high pressure amorphous carbon bearings in three dimensions. Bearing is grounded by offset steel columns connected through panzerholz blocks from (slate) tabletop to platter board. Platter bearing noise is nil.

Outboard belt drive with a flywheel allows excellent isolation of the motor. I say, "Let's have it all !"

Actually, a huge "THANK YOU" to all three of the above, who guided me to the above design. Also to the late Tom Fletcher. I certainly could not have done it without you.

Fantastic input from all the members. Challenging each other but I am learning a lot at the same time. I am still thinking of how to handle this..

Outboard belt drive with a flywheel allows excellent isolation of the motor. I say, "Let’s have it all !"
Actually, a huge "THANK YOU" to all three of the above, who guided me to the above design. Also to the late Tom Fletcher. I certainly could not have done it without you

I don’t mean to pick a fight, but your post is self-congratulatory, don’t you think?
There are those who would argue-correctly imho-that there is no such thing as perfect vinyl playback and not one option provides "it all", particularly any design that is belt drive.
As between idler, dd, and belt, belt is the worst in terms of speed stability and musical propulsiveness/dynamics. This explains why VPI plays around with using not just one, but two, and even three belts. This explains why some belt drive designers incorporate dental floss or non-stretch thread to drive the platter rather than a rubber belt. The very same material that decouples the motor through elasticity and vibration absorption-rubber-introduces other sins that are arguably worse.

I agree with @halcro , it is essential to isolate the tonearm from the platter bearing and the motor.

Among TD124 cognoscenti, this is a subject of debate. According to Greg Metz of STS who studied the TD124 in Switzerland under an original engineer involved in the design of that iconic table, coupling to and not isolating the tonearm from the bearing/motor is essential to the design. This is why the TD124 chassis encircles the armboard mount and the armboard is to be tightly screwed down to the cast iron chassis-the same chassis upon which the platter bearing and motor are mounted. The idea if I understood Greg correctly is that everything vibrating is concert is better than vibrating out of synch. The same school of TD124 experts state that a minimal mass plinth-really nothing more than a frame-similar to the stock base is best for the very same reason-a heavy solid plinth decouples the motor from the tonearm and would be the reverse what was contemplated by the engineers for best possible sound. So I am only saying that there are no absolutes. It all depends upon the design.
And yes, many people rave about the sound of their TD124’s mounted in slate and other high mass plinths. But how do we know that they have done valid comparisons? How do we know they are not judging based upon their eyeballs rather than being unbiased? The stock hollow framed base for the TD124 sure does not appear nearly as impressive as a huge hunk of slate, granite, or solid birch ply. IMHO, even the sorely missed Art Dudley got things wrong on his TD124 in this regard. He made an assumption and not an empirical decision as to what is best for the TD124. I say all of this because I used to have a heavy birch ply plinth for my hot-rodded restored TD124 and now have a hollow framed base not too dissimilar from what came stock back in 1959 (see pic in my profile of my present TD124 set-up). 

I agree with @halcro , it is essential to isolate the tonearm from the platter bearing and the motor.

This opens the turntable to colorations. Apparently Thorens has this sorted out:

The idea if I understood Greg correctly is that everything vibrating is concert is better than vibrating out of synch.

This is why the tonearm must be rigidly coupled to the platter bearing, and hopefully the surface of the platter as a result.