How to set AC balance using the 10K trimmer between the PI plates:

Hook up a suitable dummy load to the amp (or get out your earplugs!).
Apply a sine wave signal to the amplifier input that gives a reading of
about 20 volts AC measured at the point where the bias voltage is applied
to the output tube grids.  I like using a 400Hz tone, but others may do.
Some people use pink noise.  I stay away from higer tones, as they may
have more harmonic content, and 400Hz is withinin the guitar's range.
Adjust the trimmer until the voltage is equal on  both grids.  I like
20volts or a little less because my Fluke gives 2 decimal points up to
20volts.

You have just set the AC balance. 

I have heard from people who hook up the output of the transformer to a
scope and adjust the balance until the positive and negative halves of
the waveform are equal in voltage.  That method looks at the PI, output
tubes, and transformer as a complete system.  Overkill, IMHO, but some
think AC balance controls are overkill. 

This insures that the output tubes are both being driven by the same
voltage (symmetrical drive).