Spent several hundred dollars and half a day today with no result.  I bought a pair of ERA 100s and tried to add them as surrounds to my Arc and sub  set up.  
 

I set tup the speakers with not problem individually,  then went to add as surrounds to my living room.  The system asked me to select one of the era 100s. identified  which side of the room it is on using a chime sound and then selected to add them.   Tried about 30 times.  Re starting the roster, the speakers, the arc, everything.  Even a couple of times it said they were successfully set up,  but they weren't.  Spent about 2 hours chatting to a Sonos support person online.   She basically gave up and gave me a list of questions to ask my internet service provider.  That is not going to be easy - their tech support is not exactly straightforward to deal with.  Very frustraded,   Ton between holding on and seeing if it resolves itself and driving the speakers back to the shop in anger.

i have had era 100s as surrounds on this same system before,  I removed them and put them in another room as a stared pair.  The problems I had removing them should have been the red flag that stopped me trying this!

anyone she's been able to resolve this issue ?