The fact that they felt the need to run 2! 5U4’s , leads me to believe that the PS can supply > 200ma of b+

The only question is how much filament power is available. The filament string shown, will draw a bit less then 3A, but it continues off to power SK2-Sk3. So there is as a guess at least 1A more to be had.

I think it’s doable, with a little caution.

Before you start , try running the stage mono full range to get a feel for the output transformer. Lift the .022 cap from the 15K resistor and feed your signal there…John