Unity gain seems aesthetically pleasing to me. I made new measurements.
For listening I have the LOW FREQ LEVEL control set to around -8. For new adjustments I set it to zero. With each channel being driven by 18 mV I measured 21 mV output.
With the right-channel R30 already dialed fully CCW the only way for me to achieve unity gain was to increase the ovarall amplifier feedback via R23. Dialing each channel's potentiometer a little clockwise gave me unity.
With this done I still have to set the LOW FREQ LEVEL to around -8 to achieve a flat frequency response as determined by sound pressure readings using an app on my phone. I think that's odd. It's as if the woofer towers are vastly more efficient than the high-frequency panels.