I have BL5 and BL3 with a non-B&O processor.
All are connected to the processor through LINE IN. Use the surround set-up test tone in the processor to set the same level on each speaker. This gives you a working reference for the rear channels, as well as a volume level for surround. But as you say, s/pdif plays much louder than the line in.
Either try to match that by increasing the analog output from the source (the s/pdif is usually at one level), or do what I did. Once I had my reference level for the speakers, I just reduced the volume of the BL5s by 9 dB (nine ticks of the volume control) before storing the BL5 volume.
That's a good starting level for my s/pdif, which is what I listen to most of the time. When I want to listen to a movie in surround, I just give the BL5s nine quick ticks up.
9dB is the difference I have set here, you may find that the processor you choose has a smaller difference between analog and s/pdif.