401 birch or slate?

Panzerholz? Looks like a lot of hassle.

Panzerholz? Looks like a lot of hassle.

Depends if your going to buy it or fabricate it yourself.

Its usually found in a specific layer as opposed to a multiple stacked
construction.

Looks like I'll have to import a slate plinth from the UK. Not much interest on this side of the pond and PA-based Oswald Mills start at a ludicrous $5000. 

@noromance
From personal experience the SMD Quartz plinths built in the UK are very good. However, since you appear to have a reasonable plinth already, I would suggest you consider doing the following -

• Large platter & bearing from Classic Turntables - this improved the timing and coherence from the 401 substantially.
  
• Idler wheel from Artisan Fidelity - this added a significant level of refinement, particularly in the high end
  
• Power Supply from Classic Turntables - significant increase in resolution and high frequency purity
  

These mods I heard in a system I know well and can highly recommend. They took a fully refurbished and blueprinted 401 to another level by a considerable margin.