Input Impedance Measurement

To get some idea of that value you could measure straight in DC resistance with a VOM, but be sure that power is turned off or you'll DC your amp & speakers.
You probably don't have an impedance bridge, but if you do have access to a 1kHz audio oscillator (set for ~1 volt output) & a high-impedance (1 meg-ohm or better) AC voltmeter then you can trial & error measure the voltage drop across an external series resistor into the amp's input stage. Start with the 20K value they gave you & go incrementally higher / lower until you find the value where voltage drop across the external resistor = voltage drop across the amp's input. That is your AC input impedance.

952 ohms for your parallel network above; a 5% error. May or may not be significant for the particular application.

Hi Li thanks for the compliment; I could have suggested that method, which is similar to the way I sometimes measure speaker impedance curves. I just didn't want to attempt teaching ohm's law on-thread. Your approach is certainly valid & in fact a quicker avenue. Good work Mon!