The PI, I feel, is actually rather sensitive depending on your amp's volume design. I see big differences when I swap out tubes, but it is also the one that seems to benefit most from retaining whatever it is the manufacturer/builder puts in there. I wonder if it's because the amp builder tunes the amp to a certain set of new production tubes. The PI is the one slot where I seem to most often just leave it as is after tons of swapping. While every other slot has been swapped out for NOS.