I tried, just for the heck of it, tapping into the upper pins on those transformers (using my intermittent mainboard). It got output sound to the powered speakers, but it Wasn’t line level. It was speaker level and so it was noisy and too loud for the speaker inputs. Disconnected now.

Maybe I should just get a speaker-level-to-line-level converter for this, if I can find one that isn’t a car-audio device.