Resistor circuit to correct impedance matching and attenuate signal by 10 decibels
The 50 K resistor (Reference Designators - ?) is in the GND connection between the source and the load.  This is almost never a good idea in audio circuits.  Also, because your path lengths are trivially short compared to the wavelengths involved, true impedance matching is not at all necessary.  Better to go with a more simple L-pad, a two-resistor attenuator.
Why does the amp &quot;need&quot; to see a 10 K load a max value?  Most amp outputs care only about the load impedance being too low, not too high.  The line output from the amp probably is a resistor in series with the main output.  Combined with the input impedance of whatever is connected to it, it creates an attenuator, but that is only an issue if the thing plugged in has a relatively low impedance, such as headphones.  The main reason for the build-out resistance is to prevent a brief dead short across the amp output when something is plugged into the connector.  Of course, this is all guesswork because we don't have any information about the amp.
I recommend a 10K series resistor and 4.7K shunt resistor between the amp and the pedal.  This presents a load impedance that is greater than 10 times the amp's output impedance, and a source impedance that is less than 10 times the pedal's input impedance.