@Jescam

Using Era speakers with the original sonos audio usb c adapter they are able to get an analogue audio line in signal to the complete Sonos system. Just connect any pre out or line out of the Denon with the Sonos adapter. 

Thanks Schlumpf! You mean https://www.sonos.com/en/shop/sonos-line-in-adapter right?

The Denon looks like being an amp to me. Amps are not meant to be connected to amplified speakers like the Era’s. If you’d want to play sound from the amp the way Denon and Sonos meant it you’d need passive speakers for the Denon and a Sonos Port to connect to it to get the sound a different Sonos set up.

@Jescam 

Not sure what exactly you like to do… I understood you as you want to buy some passive speakers for the Denon AND an Era speaker that should be able to play audio from the Denon. 
Maybe you can tell us about your intention. 

You can connect the SONOS USB adapter to the DENON’s RECORDER AUDIO OUT and play whatever the DENON is playing through the 300 and/or any other SONOS players on your network. Note that there will be a 75ms delay for output from any SONOS units. This is not a major issue -- unless the SONOS speakers are in the same room as your DENON driven speakers.

Without @Jescam telling us what he wants to do in detail it‘s like some sort of quizz show.

For most needs there also is a solution without a delay, if the Amp has got a pre out or rec selector. 

Lol! Yes I guess schlumpf is right! Ok essentially I'm looking to replace my existing wired JBL speakers and I'm looking at alternatives. I was wondering whether wireless speakers like the sonos era 300s might be an option. Hope that clarifies. From the previous replies it seems that it is technically possible but not everyone seems to agree on the wisdom of that choice. Am i right in assuming that?

Only you will know what sounds “”best” (to you). If you go with wireless speakers, the DENON is not required. If you have a turntable connected to the DENON, you may need a phono preamp if the turntable does not incorporate an internal preamplifier.

As @Jescam said he wants to use his turntable AND CD player both. So he can use the Denon reduced to an analogue switch function. If the turntable has got a line out with build in pre amp, I would consider taking a small 2x1 analogue switch instead of the Denon. But replacing the Denon with a switch plus a pre amp imho isn’t a big advantage. 

Why would you buy a new amp, only to use it as a glorified switch, though?

@Jescam just has got that amp… I wouldn’t have bought it without the need of powering passive speakers. If he can sell the Denon of course there will be smaller options to get the setup working.