Figured it out.
I had the wrong value resistor at R18. The right value is 470k ohm.
Interestingly, my second kit came with two 470 ohm resistors per the BOM. There should only be one.
When it came time to install R18, I mistakenly used the extra 470 ohm in its place.
I did a 'stare and compare' between my first build and second, and with the BOM, was able to confirm 470k was the proper resistor for R18. Clearly, I had the wrong resistor installed.
Sound test completed: only a .5dB level difference, with a pretty much exact tonal response through both.