You’re missing my point – iTunes worked but Firefly didn’t on the M1000 and Firefly worked on the other Soundbridges (they streamed fine). However I rebooted the SB and that didn’t cure it suggesting a Firefly issue.
Therefore either the bonjour stack was messed up (unlikely since iTunes worked) or something between the SB and Firefly was messed up whereby a new ‘announce’ broadcast wasn’t handled correctly e.g. you cache the list of connected devices and, if a ‘announce’ arrives when you haven’t had the ‘goodbye’ it isn’t being added correctly and hence the new device never completes the detection phase.
I imagine that it is something like this since the SB had clearly suffered a connection loss in some way shape or form.