Duroch are metalized film capacitors - metalized aluminum film. They are ok but far from great.
PL used to use SCR caps (also metalized film) from France but switched to Duroch. The Jupiter copper foil caps are a ton better. Much more organic sounding. Much better imaging; less smear.
If you want to change the caps the first one to change are the 0.1uf film caps filtering the voltage to the input tubes.