Here's the link. Keep in mind at the time GT didn't have a 5751 available, and now they're using Sovteks which suck in comparison to the NOS.
http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf
And blame the Cesar Diaz connection for the 5751 going in the PI. He's where I originally got that idea. Less gain than a 12AX7 but same output = no volume reduction = tightening the signal hitting the power tubes. Less gain = less noise. Less gain added at the same output = less alteration to your tone. Done.
Whereas a 12AT7 will reduce volume as well because it has less output than the 12AX7.
So you've got your signal chain --
guitar > amp > 12AX7 (probably HG) > 12AX7 * 2 (module) > Loop (12AX7 for send/return) > Master Volume > Phase Inverter > Power tubes
if you've got a little noise in the signal, even though you may not notice it with a clean preamp, when you run that through a high gain module that noise will get amplified every single step of the way. Then you get to the Phase Inverter which has to take and invert that signal and send it to the power tubes. Now if you drop a high gain tube in this position, and if you turn up your master volume high enough you've actually got this tube doing double duty: it's flipping the signal AND adding another gain stage. By this stage you can have a ton of noise.
The 5751 being of lower gain but the same output as the 12AX7 allows the PI to more clearly flip the signal without adding gain to it. Hence it just amplifies that noise less.
I first came across this solution when I was trying to bring my 5150 II under control. I wanted more volume with less squeal.
I don't know if I really understand the theory behind it, but bottom line is that it works.
