You know, I've gone back and listened again through headphones via a direct connection digital audio setup and this sounds exactly like digital interference ... which is surprising if the connections are ethernet (meaning it should have electrostatic and electromagnetic isolation via transformer).
This doesn't sound like normal "noise floor" stuff, rather the voltage spikes of the bitstream itself are being amplified ... which maybe means the ethernet connection between 21 and 22 which is sending a digital signal for conversion in 21's DAC is being picked up after reception in 21's DAC/amp and amplified ...
very curious.
not a great vote of confidence in the Bowers & Wilkins name but, hey, they were just acquired so the previous owners probably don't give a shit.