The output transformer has ~4ohm output! (3.7Ohm to be more specific)

You need to connect the speaker for this mode between the 8ohm positive (which will be the “new” GND) and between 16ohm positive (will be the “new” 4ohm +)

Measurements + Pin out:

The below current mirror choke, along the Rodenhuis book, should be something similar, just split, (S6-S6a, S4-4a). While resistance is similar, the 8H is –> ~ 60H@50Hz!