Hi @derobert.

Welcome to the Sonos community and thanks for reaching out to us. I understand that you wanted to know if your Sonos Play 1 would be compatible with a Sonos ZP 90. Let me help you out with that.

I would like to share with you the Sonos S2 compatibility guide. This guide shows that the Sonos ZP 90 is S1 only compatible Sonos device while the Sonos Play 1 is compatible with both S1 and S2 Sonos App. if your Sonos Play 1 is still running on the Sonos S1 app, yes the Sonos ZP90 will be compatible and will work with your Sonos Play1. On the other hand, if your Sonos Play 1 is already running on the Sonos S2 app, they are no longer compatible as the Sonos ZP90 is not compatible with the S2 app.

I hope this helps.
Please let us know if you still have further questions or concerns. We are always here to help.

Thanks,

Yes. Provided it isn't faulty and wasn't disabled during the brief spell during which the trade up program required this. 

You would have to use the S1 Sonos OS, and so if the Play:1 has already been updated to S2 you may have problems. 

Thank you for answers @Paul A , @John B . Yes, my Sonos Play:1 speaker was already connected to Sonos S2 app so it is apparently upgraded to new system.

EDIT: I just red on sonos forums that If i have S1 setup i can downgrade S2 speakers after factory resetting them and connecting back to S1 system. So If I create a new setup with ZP90 and factory reset my S2 speaker i should be able to downgrade it while connecting to ZP90? Can a new system be setup with only ZP90 in it?

You can make a new system with just the ZP90 on it, sure. Use the S1 app.

You could them factory-reset the Play:1 and add it to the S1-running ZP90 system if you want to.

Note this will lose all configuration and playlists on both systems.

Thank you @controlav for confirming. I will try to do it this way then.