Surround Placement

My suggestion is to wall mount behind the Sofa to extreme left and right as much as possible. Sonos doesn’t sell “regular on-wall surrounds” driven by an Amp...whatever they are 😂. Sonos does sell In-wall speakers by Sonance.

Wall Mount Instructions for Era 100

• https://www.sonos.com/en-us/guides/era100

Wall Mount Instructions for Era 300

• https://www.sonos.com/en-us/guides/era300

Ok, if Era300 then on the back wall, upside down, and close to the ceiling?  
 

I know Sonos doesn’t sell on-wall surrounds.  That’s why I said maybe going with 3rd party surrounds with a Sonos Amp?  Maybe I should just add those for a clean install.  Would I just put 2 speakers over the sofa?
 

My apologies as I was speaking of the “on-wal surrounds” in jest. To answer your question…

Era 300’s would an excellent choice. As to whether or not you need to invert them for mounting depends upon high they need to be mounted on the wall to void interfering with seated individuals. Typically 6 feet above floor level as long as you don’t encroach upon the 2 foot recommended ceiling clearance.

The 300s are going to give you Atmos surrounds while almost every other option won't.

100s have very limited Atmos compared to the 300s and wouldn't be on my list of preferred options. I am using them here for other reasons and while they are acceptable they are a distant second choice to the 300s.

As to mounting,  make some temporary stands and try different heights and inverted.

All good, I was just commenting on the various options out there.

1. Era 300 with maybe suboptimal placement
2. Sonance in-ceiling speakers above the listening position connected to Amp
3. 3-rd party in-ceiling or on-wall connected to Amp

From a WAF perspective, option 2 is the best, followed by 3.  Era 300’s are not small and might look a bit wonky mounted over a sofa on the back wall. The basement ceiling is 8’.

The question is what is going to sound better, Era300 or Sonance?  I think your suggestion is to put the Era’s above the sofa on the back wall pointing at the screen (upside down so the Atmos drivers are down firing), right? Shown here in green.  

Would that be true if I ran Sonance in-ceiling speakers connected to an Amp and paired with the Arc Ultra? 

The Era 300’s are the only speakers that will allow Full Immersion when used with the Arc Ultra for Dolby Atmos.

Using stands as suggested by ​@Stanley_4 is OK to compare Era 100’s and Era 300’s. You can’t accurately compare in-ceiling or in-wall speakers using stands as they need to be fully enclosed for optimal performance.

I’ve the Era 300 just in the corners of the sofa (which is placed in a wall like you) more or less at the head’s level and they perform great. I turned them 20 degrees to the listener position.

In your case perhaps the wall mounted could be the best solution, but you can also try with the stands. For the corner it will fit, but the other side perhaps it’s not nice for you to see it while you enter in the living room. 

I don’t see the ceiling solution as the best option to enjoy surround…

The Amp is only two channels, no matter what it is hooked to that is all it can provide.

The 300s are multi channel Atmos speakers, check the store page to see all the details and pictures. Yes, channel is the wrong word here but it is close enough.

That is my dilemma, I don’t have all of these speakers to test with.  

The room has a back wall but only once side wall.  The other wall is not ideal as it’s where you walk and having a big speaker 6’ off the ground right there is not workable unfortunately.  

The ceiling solution is maybe not the best, but I’m starting to think it may be my only option.  When the Amp and two speakers are paired with the Arc Ultra, what audio channels go to the surrounds?  Only the left/right surround channels or do the various other Atmos channels also get sent to the surrounds?