How to convert 4 wire speaker level output to 3 wire line level
R1 and R2 no longer should be equal in the balanced-in/unbalanced-out circuit. As a matter of fact, R2 should completely disappear, its purpose in the balanced-in/balanced-out circuit (along with C2 that also disappeared) was to keep the balanced signals differential aspect (i.e. not ground referenced) in both input and output.

The author of that answer placed R2 there so that one could notice circuit simmetries, but perhaps he did not put much thought into possible ambiguities. I'm not an audio engineer like the author of that answer, but from my understanding, it is safe to ignore R2 existance and place a single R3 of 680 ohms. The author even states: these [R2 and R3] could (and should) be replaced with a single resistor.