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

Steve will tell you that the swiveling arm board is a compromise and is not optimum. It is a useful convenience feature. A very useful and very convenient feature but still a compromise. For optimum performance, he advocates mounting the tonearm directly to the plinth. No replaceable cut-outs, no extensions, fixed or rotating

For Garrard or for Direct Drive too ? 
Such plinth is easy to made if we already have one tonearm to mount. 

We were discussing the Garrard 301 at the Axpona 2019 room featuring his turntable, a 301 on a Dobbins plinth with a Reed arm and a top of the line VdH cartridge and Magico speakers. I don't recall the electronics in that room. The room sponsor was Van den Hul's former distributor who has since then been replaced with VPI. 
My opinion though, fwiw, would not change with any type of drive system. 

I see, the goal of Reed is that a mounting hole is not needed under this tonearm, it can be screwed to the flat surface with 3 mounting screws from the top.

Here is the engineering principle that must be observed when designing a plinth for a turntable:

The plinth must be as rigid and as acoustically dead (damped) as possible. The mounting of the platter bearing in the plinth will be thus coupled as rigidly as possible to the mounting of the tonearm. If it is not, any vibration at all can be interpreted by the pickup (arm and cartridge) as a coloration.

IOW, if the arm and surface of the platter are able to vibrate at all, if they are always in the same plane of vibration, the pickup will not be able to pickup noise or coloration on that account. So a separate arm tower is a violation of this principle and induces coloration;  the same is true of a separate arm board. If the arm board employs damping and the plinth does not you'll get a coloration. It all simply has to be as rigid as possible and damped.

For the reason atmasphere just stated, I elected to have an armboard for my VPI HW-19 made from the same material at the top plate of the floating subchassis: acrylic. Delrin is harder and better damped than acrylic, but an armboard made of it will vibrate differently that does the acrylic plate, setting up another division between arm and main bearing/platter, and therefore between cartridge and LP (where the road meets the rubber ;-). Both the acrylic armboard and subchassis top plate are very firmly secure to the stainless steel bottom plate, minimizing relative movement between the two.