What I would consider doing is to "move the controls" or the ones you care about. i.e. replace the integral control with a panel mounted one and hide it in a "rack case".
Input and output Z of the audio inputs in consumer stuff is generally misunderstood. Commercial audio uses 600 ohms in/out and balanced differential signally with twisted pair cables. The 600 matches the characteristic impedance of the wire. The "right way".
In consumer audio, the output has to be much less than the input Z. 100 ohms vs 10K in the above example. The 100 helps to eliminate ground loops., but there is a slight voltage divider effect with the 100 and 10K..
So, your suggestion would be to replace and relocate the "output level" pot with a comparable valued log pot panel mounted the way I was planning on mounting my 10k pot? Unfortunately, I don't think that would have any impact on the sub-woofer output.
With regards the input and output level difference, are you saying that the input impedance being 20k ohm versus 100 ohm output impedance is necessary for eliminating ground loops? I'm not familiar with voltage divider effects, so I'm not clear on what your final statement impacts.