An extremely thorough investigation.

Clearing up the last point first, a controller will show a WiFi SSID as not ‘in use’ -- and hence removable -- if it can’t see any active devices actually using it. So that looks normal.

I’m sorry to say that my money is still on the way the PLE is handling its WiFi segment, and whether it’s permitting multicast traffic to enter/leave it. Do you happen to have a spare standalone access point available, or an old router you could quickly re-purpose as an AP? It would be interesting to configure that with the correct SSID, wire it to the PLE, and disable the PLE’s own WiFi.

It just so happens that I do and will try to set it up later today.  I’ll get back to you.

Well, hats off to you Ratty!

I followed your advice by disabling wifi on the existing PLE, connecting a spare router to one of its ethernet sockets and configuring the router as an AP, which looks to have completely solved the problem.  I only have the slightest reservation, since the system did also work this morning with the PLE, but nothing like to the same extent.  As things are right now, I can now fully control all players in all 4 permutations of product/location, which I certainly could not do before.

So the question now is how to set things up on a long-term basis, since the new router is not the most attractive of devices and I will no doubt meet resistance from a higher authority, if it needs be in full view.  Also, as I currently use 2 wifi PLE’s, I will still have to buy more kit to implement a complete fix.  Since the problem now looks to be a limitation of the PLE, do you know of any other PLE’s that might handle multicast better and be a less intrusive solution aesthetically?

I note that you tried Devolo Magics before and I, too, had a set on trial, but sent them back when they didn’t improve my situation.  Mind you, at that time I was principally interested in finding an extender that would support a power line that crossed over a consumer unit and they didn’t help with that at all.

Anyway, thanks very much for your advice and guidance and anything further that you or anyone else can add about other PLE’s is most welcome. 

The Devolo equivalent might work, but I haven’t looked into it in a long while.

However that would mean the wholesale replacement of all your TP-Link adapters. Can you not find a discreet little wall-plug based AP which would be domestically acceptable? You only need 2.4GHz so the TL-WA850RE might do the trick.

Note that some switched-mode power supplies (which the ‘WA850RE doubtless contains) can suppress the powerline RF carrier in their vicinity, so plugging it in somewhere other than an adjacent socket might be a good idea.

Ratty, Could I get you to clarify a couple of things for me please:

I’d be quite happy to buy something like the WA850RE, but if I understand what it does, it merely passes on the wifi signal from the router and can do this only once i.e. these devices cannot piggyback the signal from one device to another.  Presumably if it were to do this, it would be classed as a mesh system?  Given the house structure, my wifi is quite poor and I’m not sure I can get everywhere in a single hop.

If, therefore, I need to persist with wifi PLE’s and just upgrade my existing ones, what is the feature that is currently missing that has given rise to my current problem, i.e. what should I look for or ask for in a new device to have the best chance of making it work as well as your suggestion with the wifi access point that I added?

You mention switched-mode power supplies and the problems they can cause PLE’s.  What are these and how do I know if a device is using one?  One of the problems I have had with my PLE’s is that they can drop out for no apparent reason and it seems they won’t come back until they feel like it.  I had this recently with my BT Vision box, connected to the router via a PLE.  When it dropped recently, the only way I got it come back after a couple of days was when, out of pure desperation, I moved the Vision power supply to a nearby extension cable, instead of being adjacent to the PLE in a double socket.  Is that the sort of problem you are referring to?

Finally, I turned off the 5 GHz band on the router, because it was felt it could be contributing to my original problem.  If that is not now the case, is there any reason for me not to switch 5GHz back on to be used by other devices, even if not by Sonos e.g. most Apple devices, will I think use 5GHz?

Many thanks.
  

Sorry, I was referring to the use of, for example, the WA850RE as an access point, wired to your existing PLE infrastructure. Sonos doesn’t support WiFi extender/repeaters, and the system would in fact object if it detects one when grouping rooms. 

I’m unable to advise on meshed WiFi systems never having touched one. 

If you did want to replace your PLEs with another make I think you’d find it quite hard to check that they’d forward multicast traffic transparently. It’s unlikely to be explicitly mentioned in the spec. I suspect it would be a case of just trying it. 

On the PLE interaction with other nearby electrical devices, it’s a known fact that their claimed data rate is obtainable only under ideal conditions. In real life the condition and length of the electrical wiring is a major factor, as is the influence of mains-borne noise from the outside world and from local equipment. Sometimes missed is the effect of ‘wall-wart’ power supplies plugged in close to a PLE. They can markedly drag down the strength of the high frequency carrier on which the powerline data is encoded, reducing the SNR and lowering the data rate. In the worst case they can kill the connection, which sounds like what you experienced with the BT box. (Our BT box also uses powerline, but its power supply is plugged directly into the Devolo PLE which has a pass-thru mains socket.)

On the 5GHz, yes feel free to use it.