Myles-A question that I have been wondering about.
If the PI isn't part of the tone generating stage of an amp, then it would seem that any specified tube would sound just like any other tube, (balanced of course!) but changing the PI from, say, a 12ax7C to a 12AX7M or, short plate to long plate, the tone seems to change- is this an aural illusion, or does the PI indeed have an effect on overall tone? or is about how a certain tube passes current? or, am I way off base here?