I don't have the headphone board or the transformer, however, on an Aero City I do have the harness from the bottom of the cab up to the control panel.
On the bottom of the cab, it's a male 3-pin Mate 'n Lok connector with male pins. On the control panel side it's a 3-pin JST NH connector.
Pinout is the same on both ends:
pin 1 - GND
pin 2 - SPK+ (jamma pin 10)
pin 3 - SPK+ (jamma pin 10)
As the speakers are wired in series, both pin 2 and 3 will show continuity with SPK- too.
Some things to consider, 1) the headphone harness is wired in parallel with the speakers, and 2) the audio coming from the harness is, of course, amplified. What does the underside of the headphone board look like?