Block diagram shows the two Powerlink outputs are simply wired in parallel on the BS9000. So noise generated by a faulty speaker *might* propagate to the other cable, regardless of whether they are daisy-chained or not — it’s just a dumb bus, not separate output drivers. Start by disconnecting the aftermarket MK.2 (thick, 8-wire) cable and set it aside. Then test: Run the BS9000 with just one BL3 at a time, and use the MK.3 (thin, 4-wire incl. ground/shield) cable for that single speaker. You’ll quickly discover if one speaker is whining, or the BS9000 is. Don’t forget to try both channels with each single speaker, using the L/R switch.
(N.B. I have never personally experienced a high-pitched whine, only the well-known click&buzz when the volume control was pressed, from using 8-wire cables.)
[Edit: You might also try plugging in headphones and see if your whine sounds on them. Not sure what that result would mean, but someone else will know.]