I have found the service manual and did finally find the actual pinout. Still have to think about this some more, but pins 6 and 7 just carry the control voltage of 30 V. This is obviously incompatible with modern trigger outputs, but that's not much of an issue. The speakers can be switched on manually.
Pin 2 is always (?) the ground pin with DIN plugs. Pins 3 and 5 carry the hot right and left signal. Did you get any cables with these speakers? If you already have at least one DIN cable to connect both speakers you just need a new one for connecting the Ultra to one speakers. I don't think that any adapters are readily available fro this very special use case.