Hi @Mrcgibb
Welcome to the Sonos Community!
Perhaps you could provide a picture of where you’ll be sitting, to give us a better idea of your speaker arrangements?
Rear speakers should definitely be placed behind the listeners head, however - if this is not possible, anything else is definitely a compromise, unfortunately. And Era 300s should be placed squarely, pointing towards the wall the TV is against, and not angled to point either at the TV, or at the listening position.
Perhaps the best option would be to buy them and try them - as you are in Europe, you will get 100 days to try things out and get a full refund if you are not satisfied - assuming you buy direct from our online store. We will even cover the shipping costs of the return for you, should you decide not to keep them.
Please bear in mind that the Era 300s need space around them to work effectively - they will not be as effective on a bookshelf, for example, where they would be somewhat enclosed. Full guidance can be found in the Era 300 product guide.
I hope this helps.
I have just went from Ones to ERA 300s as surrounds. The difference is definitely material, and my setup with the Arc+ones+sub with Atmos was already good.
Note that it seems many people are trying to play compressed quality tracks to compare, try it with proper sources (e.g. TrueHD based Atmos). It is stunning.