In fact the Mod is not such a big deal, basically going from .22 to .33r emitter resistors.

What I remember seeing on the schematic (quoting from memory, too lazy/sleepy to open it now, it's 6 a.m. here)was thgat the Vbe multiplier transistor had in parallel with the BC and BE resistors another two, much higher value.

I've seen that on a few amps, they do it so as to save the adjustment trimmer $$$$, so they set the bias to a general purpose correct value, but the board tester (still at the factory) has the option, if he sees bias current way too high or too low beyond a certain range, to clip one of those resistors off.

(Slightly) better than nothing, but it's a one shot gun, if later you replace transistors or simply got it the wrong way, adjustment involves pulling the old resistor leads , fitting a new one (which might not be the required value anyway), etc.

Rising ballast resistor values does not correct this basic flaw but helps a little.

Of course the proper solution is to replace one of the biasing resistors with a trimmer and set the values properly.