If Sonos support have said that, I doubt if anyone here can help.  The only thing I could recommend is to leave the device powered off for some time and then try a factory reset again.  I take it the device doesn’t show within the Sonos App>Settings>About?

That’s what I’m afraid of too and generally surprised that there’s no actual button for a hard reset should this happen or that the two options they left me with - replacement or upgrade discount of 15% from Sonos.com - will net me the same cost I can find at most retail stores today for a new product. For a high-end device that set me back 700eur and I had not fault in bricking this is all very disappointing to say the least. 

Sorry to ask a stupid question but just to confirm you followed these instructions to reset: Reset your Sonos product | Sonos

And as a last resort, please look at this article(although I doubt this will be the reason but it’s worth a try...) Tips & Tricks - Resolving random issues impacting Sonos devices.. | Sonos Community

Just tried that too - reset the wifi and followed the instructions but still not working. I bought a subwoofer (gen 3) a few days before and thought maybe this could be the cause of it but nothing I’ve tried is helping unfortunately. Time to recycle it now… Thanks for your help and quick replies nonetheless!

You may perhaps want to keep an eye out at the link here (below) for any Sonos HT refurbished products that come at a reduced price, with the manufacturers guarantee and/or check online to see if there are any electrical repair businesses in your local area that may offer repair on Sonos devices too.

https://www.sonos.com/fr-ch/shop/certified-refurbished

Hope that assists.