Since me the monitoring tone on my GM-15 und GM-10 was much too loud, I made a little modification in the AF section.
Into the circuit path of U3, pin 4 to the AF switch Q1, is inserted a 100 nF capacitor. Thus one keeps the output voltage from U3 away from Q1. Thus split circuit path on the solder side directly at U3 and solder the capacitor over the interruption.
Subsequently, join the source of Q1 over a 1 MΩ resistor with V+. For it soldering the new resistor from the upper end of the vertical resistor on the right below Q1 to the upper end of the likewise vertical diode D9 (close at the PWR plug) on the component side.
Perhaps aesthetes find also a more inconspicuous place for the new resistor on the solder side.
After these modifications I could use my headphones when transmitting (HI) and QSK fully enjoy.