The output resistors have always been 47k from the factory. And the original gain of both would be the same, when using 12AX7 tubes. I'm guessing that the additional resistors you find in the 1974 unit were selected to pad the output for use in an effects loop, or possibly to patch the two channels in series for overdrive sounds. Especially if the additional resistors were only in Channel A, to provide attenuation for patching the output of Channel A to the input of Channel B and using unmodified Channel B output to drive a power amplifier.
Ground loops are tough, but using short signal cables, kept close together (to minimize the area included in the loop) and also keeping the power cords of the two units close together bundled to be the same length and plugged into adjacent power outlets. Dressing the power leads to minimize the included area in the "loop" between the signal and power grounds will reduce the ability of a magnetic field to induce much hum into the loop.
Your use of lower-gain vacuum tube 12AU7 also makes sense for use in effects loop, without overdriving the effects return input.