Luke,
Welcome to AK! Sorry about the issues with your 340 receiver. I have restored a couple of 340B's, which are the next evolution of the 340. Here's a link to the 340 schematic from the H H Scott website (kept going by some outstanding AK'ers, I believe):
http://hhscott.com/pdf/340A.pdf
It's not unusual that a given unit will not exactly match a given available schematic, as Scott often did make some changes during production. However, the website archive says that the 340 was in production for little over a year before the 340B was introduced, so I would imagine major changes were unlikely. In any case, here are a couple of questions to try and help figure this out:
1. Is the distortion in all the audio sources (eg. Aux input, tuner, phono, etc)?
2. Is it for both channels, or just one?
3. If you suspect the driver/phase inverter, have you tried new tubes in that position (or swapping if it's just one channel)?
4. What leads you to believe it's the PI that is causing the issue?
5. Has the unit been modified in any way? If so, pictures of the chassis underneath might help diagnose it. By modified, I mean changes beyond just one-for-one like substitution of components, for example replacing a 0.1 uF 600V coupler with a newer one of same value. I'm talking about things like bypassing the loudness contour controls, or something where the component values or the circuit have been changed.
6. What is different in your unit compared to the schematics you've seen (particularly the one at the link above)?
7. Was the distortion always there, or did it follow some work that was done? If so, what work?
8. Can you describe the distortion you are hearing? Is it a buzz/hum, or is the sound of music obviously distorted?
Knowing more about these things might help figure something out.
Dave