JA found out that the frequency response depends on the setting of the volume knob.
It usually is simply because the circuit behind the volume control has a capacitance so a low-pass filter is created that is frequency dependent.
This thus depends on: volume control position (-6dB is worst case), resistance value of the volume control (the lower the better), input capacitance of the circuit connected to the wiper.
Judging from the PC no extra capacitors on the input so it would only be the input FET GS capacitance and the flat-cable.
Given the fact that gain is only 2.5 x and Amir testing with 2V in and adjusting the volume to be 2V out the volume control would have been near the maximum value so smallest frequency range so in this particular case the volume control would have been close the technically 'worst' condition.