In order to function we would set R bypass less than Z gen. The filter is less effective when Z gen is 50 ohms.
Z bypass also needs to be less than Z load. Simply stated the bypass must be lower than the source and load.
You should be able to see a reaction to the input frequency.
That reaction is defined as 1 / ( 2 PI * F * C).
The last amp I built was very stable with the input shorted but would pick up strays when the input was open. Putting a low value resistor across the input helped but it would then load the tuner so a filter was called for. I used a simple L filter and it worked but this filter is used on LM386 amps so it could be a good alternate.