The phase inverter (v3) circuit splits the signal into two signals, the original and one that is opposite in phase (180 degrees), and sends these two signals to the output tubes. These two signals are used to drive the power tubes as a push-pull type amplifier.
At higher volume levels, the PI thickens the sound nicely and adds lots of sustain and harmonics. But, if the PI is driven too hard, the result is a mushy, buzzy type sound. The 1987/1959 amps don't drive the PI too hard (but the 2204/2203 amps do IMHO at volume greater than 7-8). Using a Post-PI master volume will allow you to get the sound of a driven PI at lower volumes.
