Progress?
Their engineer suggested the following changes:
R1 392
R2 472
R3 823
R4 622
R5 392
R6 822
Unfortunately, the resistor kit I bought was limited, and I don't have the 39, 62, or 82 variants. So I tried with 33, 56, 75 instead.
This is what it looked like:

I also replaced the 10k pot on the PCB with a 25k trim pot. The original pot was mechanically unhappy. It's more stable now with the trim pot. I might try and find the replacement 10k pot, but, eh, I don't know if it's worth the effort.
Since the change made some improvements, I tried changing R4 to 7.5k and that helped horizontally, but not vertically. So I tried changing R3 to 100K, and it was too much. Now I'm ordering a better resistor kit to put the suggested values in. I'm much more confident in the possibility of this working now though.