Looking inside [with the faceplate facing you] right behind the Treble control is a small board with 3 trim pots (round nickel size black and white pots) for the HRM tone stack. The from let to right are the treble, bass and mid controls.

The trim pots are adjustable using a small screw driver type tool.
WARNING: As there are dangerously high voltages in a tube amp that can kill, it is best to use a non conductive plastic adjustment tool designed for such purposes.

Then, moving further towards the rear, is a larger board that has the pre-amp parts ect.
There are 2 similar pots on there. The one just left of center is the OD trim. The one to the right near the edge of the board furthest from you is the PI trim. That will adjust the balance between the two halves of the inverter tube V3 that drives the output tubes. By changing that you should hear a difference which you can adjust to taste.

Again, adjusting pots while the amp is ON is dangerous, as there are very high voltages inside that can kill.