Possibly a variable resistor in series with one of the Phase Inverter anode resistors. It would allow adjustment of the PI outputs to allow you to balance them, or not if you prefer an imbalance.
Same idea as having 82k/100k anode resistors on Marshall's etc, but a bit more precise. It's not a new idea, and better suited to hifi/PA anyway. The Sai PA amp the sgmypod owns has this feature.
There's debate over whether a perfectly balanced PI is a good thing in a guitar amp. Most would say not, or at least thatbi doesn't make much difference.
I would concur.
I expect that it is a method of balancing the AC signal in a long-tail pair phase splitter.
This is hardly a new. The earliest schematic I've seen this feature on is the Harman Kardan Citation II from 1960! Not many guitar/ bass amps have this feature, although Ampeg used it in the SVT.
The jury is still out as to whether perfectly balanced AC drive to the power amp is of any benefit in a guitar bass amp, although my personal view is that it isn't, and that balance in most guitar amps is "good enough for rock and roll".
On a more practical note, pots/ variable resistors are relatively unreliable compared to fixed resistors, so unless there is a very good reason for using a pot here then you will ultimately be compromising the reliability of the amp.
