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 or amplifiers which only have a single gain control.

There are quite a few ways to add a volume control, from a simple variable resistor in the send / return line however these can change the sound as they change the impedance of the signal going through the chain.