Solution. Connect your KEF speakers to your BS node by Bluetooth. I have done this and it works a treat. It is not the NS node it is the KEF speakers. I had the same setup and the same problem. I tried a different input (Echo Dot) and encountered the same hum. The workaround it to connect by Bluetooth using the both the BS app and the KEF app, it is a simple Bluetooth setup.
Hope this helps
Hope this helps