Unfortunately, the schematic is not included, just the bill of materials that has some difference in the values with your schematic, especially in the resistors. Maybe because one of the two AC125 is an AC126?
In the document we can read something about the heat-sinks for the two AC128 that can get hot during the use.
Did you think about it?
this circuit had AC125, AC126, and 2x AC128.
this is seen in the schematic for the PR80:
what i have done is drafted a tagboard layout using the ‘greg fryer’ schematic, which i’m assuming was developed to ‘clone’/adapt the circuit from the PR80, using transformers that are actually available(?)
(this schematic substitutes x1 AC126 with another AC125, resulting in: 2x AC125, 2x AC128)
anyway, it’s the same schematic used for a veroboard layout that’s currently available, i found it here:
Pre-gain and master volume should be easy to add. Master volumes don't go at the end of the circuit, they usually go between the preamp and the power amp sections. In this case, I'd try a 2u coupling cap into a MV pot (experiment with the value) right after the 2nd AC125.