I want to connect a pair of Bang and Olufsen Beolab 6000 powered speakers to a Bose CD-3000 system using RCA connections between the CD-3000 and the speakers. Do the powered speakers need their own volume control or are they controlled by the Bose system volume?
The line output on the Bose is fixed level so they will need their own volume control. You can connect something like these https/www.amazon.com/MCM-CUSTOM-50-8394-CONTROL-IR/dp/B01CX82KH2
in between the Bose and the B&O speakers.
You can turn down the Bose internal speakers and it won't affect the volume of the B&O speakers.