Yes, the volume can be controlled from the Ouverture buy of course you will also be adjusting the volume in any active speakers you may have connected to the other powerlink socket on the Ouverture.
If you require indepenent control of the passive then you must use the masterlink interface together with an IR eye.
I am afraid that I do not know why you passive has no on/off indicator.
The nominal power rating is 30 watts per channel which is plenty of power to run Beovox 5000 in a reasonable sized room at reasonable volumes. It would not cope with Satus Quo in the Albert Hall!!
Where are you located, I may know a local service engineer?