I boxed everything and set it up again in Sevenoaks Guildford, who very kindly offered to investigate the loud fizzle hum from my PMC Fact 8s. Their PMCs were silent, though my SN3 purred quietly.
Back home again I was going to turn off the ‘Ethernet over mains’ devices sharing the same mains ring and socket. But no need; my speakers and amp had now become perfectly silent.
What could have fixed the problem? Merely removing and reattaching every cable? I did reinstall with the HICAP separating the SN3 from the ND5XS2, as shown in the picture. Before, the SN3 & ND5XS2 were adjacent.
Ideas welcome. It’s not enough to find a solution without identifying the problem.