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I think you're right-- thanks for picking up on this. I do still think it matters; if the CM signal is getting converted to DM, the system's CMRR will be limited, no?  Bill Whitlock of Jensen makes a big kerfuffle about balanced impedances. Here's an AES presentation that you may have seen. It's a bit goofy, but gets the point across. https://www.aes.org/standards/webinars/AES_Standards_Webinar_SC0505_20210726.pdf   If I'm being honest, I didn't realize there was a CMRR test. 😁 I'll be using this more often. It can't do sweeps and I don't think it can append results in APx500 v4, but here are some results tested at 1kHz with a 1VRMS signal.     I tabulated some data at different positions. Looks like there is a "sweet spot" with the conventional pot hookup, whereas the shunt pot does very well once you clear the first few degrees of rotation.   Position Shunt pot CMRR (dB) Series pot CMRR (dB) Min 50.800 16.024   77.772 23.607   89.748 27.03   99.328 29.009   98.410 30.802   97.917 39.849   97.656 73.718   97.362 67.308   97.228 40.310   96.271 35.341   95.442 30.717   94.715 32.493   94.031 45.187 Max 93.844 86.054

The distance from earth to AC and other components is scary small there.  I'd at least put in isolation slots as the voltage can creep over the PCB. 

Making a soft starter module with switchable voltage function. No screws required if attached to the AC socket directly.

So complicated. Like the volume mute function.

I checked the forum members if they reacted to death of Norm Macdonald. Dunno why but somehow I knew he was beloved by forum members. Only famous person I've cried over his/her death. Read his book when i was "away". Imagine doing standup and made jokes about cancer and death for 20 mins (Me Doing Standup) while you're battling. Well at least he didnt lose his battle, it was a draw.