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.

@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.
 

@noromance 
Drop in replacement. Undo 2 screws holding the top of the idler, lift out the idler wheel and drop in the replacement - literally 2 minute job. Drop a little oil on the spindle ( top and bottom ) to avoid any scouring. The bushes in the 401 were in good shape.

As an aside I have a 301 rebuild on the go and the new sintered bushes from Audio Silente look very good. I have purchased both motor bushes and idler bushes but have not installed these yet. Their idler is a good price but not in the same league as the AF.

@noromance 
Hi there,
Good to hear about your upgrades. Great looking 401 with the slate plinth.

Since my last post I have rebuilt my listening buddies AG 401 with a custom engineered quartz stone plinth, 12kg brass platter & bearing, and Kuzma 4Point11. Cartridges are Kuzma CAR50 and Van Den Hul Calibri. The Kuzma replaced a Moerch DP8 & Helius Omega. It's an excellent arm and works with the 401 beautifully.

So yeah - I have a good idea of what you've got.

I think the 401's are a bargain as far as high end tables go and there are plenty of off the shelf upgrades.

@noromance 
Should also tell you I built a 301 for a spare TT for myself and to run 78's. It blitzed my old second deck which was a Platine Verdier.

Some of my mods included -

Full motor rebuild, new bushes, blueprinted evrything internally - only runs half as hot as original and very quiet.
I replaced all the rivets connecting the linkages under the chassis with nylon bushes/teflon washers & nautical grease to eliminate most of the metal to metal contact under the chassis.
Separated the motor on/off switch from the on/off lever so that when you change records the idler wheel disengages but the motor remains running - this is a big advantage because the motor takes a while to stabilise each time you turn it off and on. It also removes 230V from under the cartridge.
Reworked the bearing and platter.
Custom built a high density compressed fiber bamboo plinth.
Replaced the rubber inserts under the chassis with carbon fibre washers. 301 is through bolted to the bamboo plinth with carbon fibre washers above and below.

There are a few other tweaks as well.

Currently running Fidelity FR64S/B60 with various cartridges on that deck. A few of my friends have been shocked by the performance capability of the old Garrard.

FWIW the 401 is a better deck for modding, it has a stronger motor - the 301 is more rube goldberg.