PPIMV –  (Post Phase Invert Master Volume)  For valve amplifiers without a master volume this is by far the best kind of master volume modification around, tonally it provides very little colouration while allowing you to crank the preamp valves.                           Important to point out though that it doesn’t overdrive the power tubes unless up loud so the distortion will be far different than using other devices such as speaker load attenuators.  This is a popular mod to carry out to older Marshalls