How many ohms is the CD input impedance of the amplifier that you regarded as LOW impedance?
The CD input passes through the Loudness, Balance and volume potentiometer before it reaches the FETs.
940R.
In stereo position, R145 (4k7) is shorted, in min loudness position, R151 is shorted too, there is only a 470R in serie.
In mid position, balance pot is shorted, volume pot in max position is shorted, subsonic filter is also shorted in defeat position, there is only another 470R resistance in serie with gate of the fet....Then, 470R + 470R = 940R....If input fet is defective, there is a low impedance between gate and source.....