Knob on earcup is for an optional mic. that attaches there...
you won't use it anyway.
So, it seems you're missing the connection box...about the size of
a pack of cigarettes. It takes a speaker level input from your amp, and
sends it to the headphones, and the box routes the signal to the speakers,
which can be switched on/off at the box. So I guess try a speaker
level output signal for starters. Good find.
