Hagen, I have found that running B&O speakers from other equipment is quite painless. I've never come across the signal drop out from not supplying the "control voltage of +5V" you speak of.
Am trying out various receivers/processors and am using a Linn Unidisk SC and a Primare receiver.
Both have analog outs for the various surround speakers, and both allow you to set the relative level between the speakers, and to control volume up-and-down.I bought RCA-to-Powerlink cables at my B&O dealer. The reason I didn't use a BeoSystem 3 is because it doesn't have S/PDIF coax out, which I need for my setup.
Have had a pair of BeoLab 5s connected via s/pdif coax to a Konnekt 24D for several months now, and have never experienced that the speakers go to sleep unless I switch them off.
Similarly with a pair of BeoLab 3s that I used as back surrounds before switching to BeoLab 4000s. Both types have never gone to sleep on me while the equipment they have been connected to has been on.