The difficulties with the 401 can be traced to market forces (Japanese TT’s on the market) and the subsequent buy out by Plessey. . The cosmetic finishing and some engineering refinements went south quite rapidly after it’s release.
The 401 is a revised 301 with a strobe light. The chassis is more rigid and better cast (As is the platter) , the idler housing more substantial, the motor casing cast iron, the motor bushes much improved, the speed disk dynamically balanced,the magnet assembly adjustable (Although ceramic = potential problems) the spindle thrust bearing is sintered and domed (better for Stereo) On paper these are plus points.
The motors have less torque and run quieter, I think for me this effects the sound, certainly there is more refinement but at the cost of, for want of a better word, ‘energy’. The cosmetic finishing of the 401 lessened throughout production. I suppose the 401 closest to the original designers intention is the ‘push button’ 401 which was the very first type
(the strobe light works by depressing the pitch control) nice cloth bound manual etc etc.
In short the 301 was playing catch up with Stereo and the 401 suffered from Market forces and a hatchet job takeover.