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

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.  

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.

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.

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. 

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. 

For practical reason this type of Steve Dobbins plinth for Garrard 301 is universal for different tonearms if you're going to use many.
Same construction in Stereophile article

Aesthetically Artisan Fidelity 301 is the best (imo), but you need many armboard to swap tonearms.

Well, I know Steve Dobbins. I have met him face to face and have had extended conversations with him about optimization of the venerable Garrard 301. 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. Why should this be surprising when extensions resemble diving boards?
Everything needs to be kept in perspective. The Garrard 301 is not the ultimate in terms of being quiet or in terms of speed accuracy. Instead, it has it's own sound, a very good one. In light of that, one should not sweat bullets about having or eschewing a rotating arm board. They are clever and indispensable for those that want to use lots of different arms. The compromise in overall SQ is likely negligible. I would argue that it is only when using the very finest MC cartridges with the 301/401 that one should avoid any otherwise avoidable compromises. 

Dear @kanchi647 : 1+  with millercarbon advise.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.

For practical reason this type of Steve Dobbins plinth for Garrard 301 is universal for different tonearms if you're going to use many. 
Same construction in Stereophile article 

Aesthetically Artisan Fidelity 301 is the best (imo), but you need many armboard to swap tonearms. 

Yes there is a difference that said is your plinth with these options 
going to be your final build/purchase or a test bed.

I built a plinth that accepts various arm lengths with multiple 
armboards. I decided the pivoting option was not for me and went with boards mounted directly to the plinth. Also various materials 
of construction for comparison.

One difference is the boards do not touch the exterior surface
and are attached to next layer below via threaded fasteners.

All sorts of opinions out there and some are easier to execute 
than others.

https://forum.audiogon.com/users/totem395

The best for me is armboard on the rails to change Pivot to Spindle distance quickly for various tonearms when needed. This design invented by Micro Seiki for Luxman PD-444, PD-555, PD-441 series back in the 70’s.

If you will look here such metal armboards mounted on the rails and can be moved left or right, they are fixed with lock. Clever design! The "plinth" of those turntables are metal (damped inside).

In modern design i see something similar on Dr.Feickert's turntables.  

Of course there is a difference. For what you are doing though the most important considerations are flexibility and ease comparing several different arms. The best way to do this is with an arm board you can rotate to easily accommodate any length arm. Also if you need different hole sizes this is the easiest way as you can make several boards identical in every way except for the hole. 

This can all be done fast and cheap because its really only for comparing. It will be more than good enough to compare. Then when you know which arm you want it can be mounted in the plinth, or on a better more finished looking arm board. 

Another thing you can do along the way is make some arm boards out of different materials. Set up right, like with a notch in the boards, you could change them out so fast and easy it will be no problem to compare several. MDF, acrylic, different wood species, you can try easily and see. They each have their own characteristic signature sound and this will enable you to tune for the best result with your final arm choice. This is a far better approach than what most guys do trusting someone else and then paying them a lot for something they have no idea what it is.

This will also teach you enough to know whether you want the arm on a board like this or in the plinth, or inlay mounted in the plinth. In other words router out an area of plinth and inlay your arm in that. Either this or the rotating arm board are great ways to have a good looking table that can easily accommodate a range of arms going forward.