I believe a better solution to the phase inverter distortion problem is to forget the grid stopper mod and simply replace the 1M grid leak resistor with a 1MA pot to create a PRE Phase Inverter Master Volume pot which will allow you to fine tune the grid stopper & grid leak resistor sizes to really control phase inverter distortion. By turning down the master volume by about 1/3 you do the same thing as adding a 470k grid stopper and 470k grid leak to the phase inverter. The nice thing about this master volume is when it's set a max the 5E3 circuit is unchanged from stock.