401 birch or slate?

@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.

Thanks @dover . This is very good stuff. I’ll look into the AF idler wheel for my birthday! Looks like I will go with slate. I have found a source in the US.
Not sure if I should open a new post but...If using slate, should I use an arm-board or mount on the plinth?

I went with slate. See my Virtual Systems entry for update.

@dover  You said "Idler wheel from Artisan Fidelity - this added a significant level of refinement, particularly in the high end"

Could you kindly elaborate in a bit more detail what you mean? I'm tempted to get this wheel. I notice there is another offering from Audiosilente in Italy for 1/4 of the price. Thanks.

@noromance 
I would describe the improvement of the AF idler wheel as similar to a cartridge upgrade - the higher frequencies were more detailed, more nuanced. There were also improvements in coherency and musical timing.
When we upgraded to the Artisan Fidelity idler wheel, the 401 had already been refurbished by Audio Grail, in other words the AF idler replaced a refurbished standard idler wheel in good condition. The 401 in question also had had an upgraded platter and bearing from Peak Audio which had yielded a massive ( not hyperbole ) improvement in musical timing.