Most likely you are simply missing one piece of kit. Like the BeoLab 2, the BeoLab 7-X speakers were intended strictly for use with a B&O master with PowerLink outputs. As such, there is no 'Line in' setting on the speaker, which means the internal amplifier never switches on (the trigger is in cluded in the PL cable).
Here in the US B&O sell an adaptor (898127) whch will supply RCA to DIN connection, and the trigger voltage to turn the speaker on. Note that unless the power supply is attached to a switched outlet the speaker is always powered up.
Russ
We kid because we love.
Bang & Olufsen Tysons Galleria
McLean, VA USA