I don't claim to have a lot of experience with different armboards for the Garrard 301 but I did recently buy a mint condition Garrard 301 and I am having a plinth built. Mine will be layers of solid cherry and birch ply. I intend to mount my Reed 3P directly to the plinth. IMHO, having a pocket under the armboard (as so many stock plinths do) is an immediate compromise. There are some fancy (read; expensive) Garrard plinths out there with multiple armboards that fold in and out from the plinth to support different length arms. Again, IMHO, this looks nifty but is a compromise. I believe that somewhere along the way, smart people figured out that with an idler, it is best to have one common resonance running through the entire plinth and tonearm mount. Otherwise, tonearm mounts would be completely isolated from the plinth. This approach-the completely isolated tonearm mount-is sometimes seen with exotic belt drive tables, but not idlers. You seem to be on that wavelength already so why do you need an armboard? The answer may be that you want to be able to swap out tonearms without a marred plinth (perfectly understandable) or that you need the extra height to achieve proper VTA, particularly if you have an after-market platter. So my own opinion-driven by reliance upon blind luck and intuition-is that different woods or materials screwed down to the plinth (why use adhesive and be, uh, stuck with it?) would have a small effect on sound.
Someone with far more experience than I (my plinth builder in the UK) says that layers of birch ply is the most neutral and that solid cherry is the most warm. I guess by layering the two, I hope to split the difference. I agree with Millercarbon that if the arm is to be mounted directly to the plinth, the plinth must be perfectly flat but it seems to me that adding an armboard screwed down to the solid plinth makes achieving a perfectly flat-relative to the platter-mounting surface even more difficult to achieve.
I might mention (and will) that I also have a TD124 and at one time I experimented with 4 or 5 different armboards including one made of aircraft grade aluminum and yes, they all sounded different but that is an entirely different deck with the chassis supporting the armboard-unlike the Garrard.
Someone with far more experience than I (my plinth builder in the UK) says that layers of birch ply is the most neutral and that solid cherry is the most warm. I guess by layering the two, I hope to split the difference. I agree with Millercarbon that if the arm is to be mounted directly to the plinth, the plinth must be perfectly flat but it seems to me that adding an armboard screwed down to the solid plinth makes achieving a perfectly flat-relative to the platter-mounting surface even more difficult to achieve.
I might mention (and will) that I also have a TD124 and at one time I experimented with 4 or 5 different armboards including one made of aircraft grade aluminum and yes, they all sounded different but that is an entirely different deck with the chassis supporting the armboard-unlike the Garrard.