Hi,
There is a specific specification of BL5 Powerlink cable. Im not 100% but I think it is heavily shielded and has a ferrite ring on each termination.
When using Powerlink only, the Powerlink transmits analogue audio signal for both channels and volume control.
When using Powerlink and SP/Dif, the BL5’s only use the Powerlink for volume control and SP/Dif for the digital audio signal.
I am not sure what your problem maybe – whether it is poor quality (for music) Powerlink cable or something else. You can test this by swapping channels on the BL5 and by swapping cables between the source and the BL5 and the cable that daisy-chains the BVL5s together.
However, I believe the quality of sound using both cable sets will superior all around. A poor quality Powerlink cable (if that is the problem?) for volume control and a good Coax cable should suffice.