Consider using the Core’s optical output to the Beolab 28’s combi-optical/analog input? (Steve @SoundsHeavenly assures us that the Core can do variable volume out over the optical. See: https://beoworld.org/forums/topic/beosound-core-beolab-18-connection/ )
It’s a cheap & easy thing to try while you wait for your Powerlink cables to be delivered. A high-quality optical splitter is important (because the Core only has 1 TOSlink output & 28s do not have optical “thru” to the other speaker), but the TOSlink-to-3.5mm optical cables are cheap. (Don’t use 3.5mm optical “adapters”, get one-piece cables; too much light loss/reflection otherwise, especially with a splitter inbetween.)
It might even sound better! Certainly it will sound different. Powerlink, being analog line-level audio, is susceptible to electrical noise, as you have discovered. Plus you’re doing a D/A conversion inside the Core (to transmit the digital source or broadcast audio over analog cables) and then an A/D conversion inside the 28s (to perform the sound correction/EQ/etc.). Even given the highest quality converters, it seems pointless. [Insert equally-pointless argument here about effectively reducing audio resolution bit-depth, by doing volume control in the digital domain. Just *try* it!!!]
P.S. What’s dumber than a $5000 Audioquest “Perfect-Surface-Silver” power cord? Gold-plated connector shields on a plastic or glass optical cable…