Your two Era 300’s have been added as Surrounds and hence should be Rears not Front speakers.  Your Arc provides the Front Left, Centre, and Front Right speakers

Ideally, the surrounds should be positioned equal distance from your seating position and slightly behind, with a slight angling inwards.

Thanks for that. Yes, I’m aware that optimally they need to be behind near ear level, and when I get my stands next week they will be. When I do, what volume level do I need on the 300’s? Also, I had them on paired stereo mode when I play music and all I did was unpair them. Are they automatically now in surround mode technically? Not withstanding the positioning. TIA.

You will need to open the Sonos App>Settings and select your Arc.  Then scroll down and select Add Surrounds.  This will bond the two Era 300’s to your Arc.  At this point you have two options:

1/ If you have an Apple device, use Trueplay tuning;

2/ If not, adjust the EQ settings and Surround Audio settings to your choice. (You won’t have a separate volume control as you will see displayed with your stereo pair)

Oh, ok. At the moment I have the Arc bonded with my sub mini, and the two 300’s separate. So I have two separate ‘living rooms’. Really appreciate your time in responding. 

Hi, yeah you need to bond the 300s back to the Arc to use them as rears. If you want to play music through them, it’s easier to play music through the whole Home theatre ‘room’ rather than keep taking them off the HT set up, and then having to add them again for films.

That’ll make life a lot easier, but will it not lessen the soundstage by losing the stereo effect for music? That said, by having in them in stereo mode for music, doesn’t that cancel out the Atmos facility on the 300’s?

If you were to use a music source like Tidal or Amazon Music Unlimited, you can play Dolby Atmos music through your Arc, Sub and Era 300’s, which sounds amazing.   The Surrounds will also play music in stereo when in a Home Cinema set-up.

Yes, I have Amazon music unlimited so I usually use it for that. So will both set-up’s still work the same in regard to Atmos? Meaning, whether I keep the system in surround mode like you’re suggesting, or change back to stereo mode with the 300’s?

As long as you don’t Group with another Sonos Room that doesn’t support Atmos, it will work great.

Ok. Thanks for all that.