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.

Generally equipment such as amplifiers have a high, non specified input impedance, commonly known as a bridging input. Input impedances range generally from 10 K Ohms to 100 K Ohms. This is generally done to allow you to parallel connect several amplifiers across your program source, and is commonly done in PA Applications. It is very common in Broadcast and other Professional applications to have equipment with an inherent high input impedance, and have a switchable 600 Ohm Termination resistor across the input terminals to give you a 600 Ohm input if this is required.
Suggest you simply ignore the amplifier input impedance, and load the input with a resistor of the value you require for your filter. If for example you need a 1000 Ohm input, simply use a 1000 Ohm resistor, and ignore the actual input impedance of the amplifier. There will be a small, but probably not significant error having the 1000 Ohm resistor in parallel with the 20000 Ohm or thereabout amplifier input impedance.

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

Bob your answer is correct but putting a 10K resistor in series with the input and feeding it with a 1V 1KHz sine will allow you to use simple ohms law. Measuring the voltage drop across the 10K to calculate the current flow, then subtract the voltage dropped across the 10K from the 1 volt input. Use R=V/I to calculate your input impedance, providing it is purely resistive. Good job Bob, I liked your answer too.