From MetroAmp Wiki
"PPI MV" is the acronym for the popular modification called a "post phase inverter master volume". Commonly installed in tube amplifiers not equipped with master volume controls.
A master volume control by definition is a control that attenuates signal level to control amplifier output. This can occur in many positions of a typical circuit. Marshall Master Volume (2203, 2204) amplifiers, for example, use a single 1M audio taper pot installed between the tone stack and the phase inverter section. While effective, this type of master volume leaves something to be desired tonally, since at settings below "10" it negates signal gain (overdrive) that occurs in the phase inverter. This oversight created the opportunity to improve the master volume circuit to include the phase inverter gain.
There are several ways to attenuate signal level after the phase inverter gain. Each solution has it's own quirks and tonal characteristics. One of the most popular circuit mods appeared in an article written by Craig Anderson in Guitar Player Magazine in the late 1970's. This circuit is also ubiquitous on the internet and has adapted many monikers, including "the Rich mod".
This circuit is diagrammed here:
A simple DIY kit for this mod is available HERE. And described in detail on the Metroamp PPI MV kit page.