Here https://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1549418998/1#1 we read the bass tightens when 0A3 in Torii Mk3 is replaced with 0D3.

This sounds like a nice fine tuning ability, and leads me to wonder:
Would a voltage regulator circuit be made variable voltage if a varistor or a stepped attenuator were placed parallel to the voltage regulator tube?

Brian