Amp/Arc Ultra set up

In a Sonos environment, when a soundbar has surround speakers added, they are “bonded” in the same way as a Sub. This means that a dedicated, one-way 5Ghz wifi link is created from soundbar to rear speakers (or Amp, in your setup). Any line-in capabilities the rear speakers have are disabled because the link is one way: there’s no link back from surrounds to soundbar. Even when Ethernet cables, any rear speaker/Amp inputs are disabled. 
So no need for wires between Arc and Amp; leave them wireless and see how the system performs. 

The sonic results will be the same - wired vs wireless. Since 5GHz does not penetrate walls very well, a wireless surround AMP should be in the same physical room as the soundbar.

If you have dropouts or lag, then try Ethernet between arc and amp

Ethernet between Arc and Amp or via router?

Been told either between them both or to shared switch, or router i guess wont matter…

Thanks all. Got rid of the wired connection between  tv and amp in favor of a wifi connection between arc and amp. This solved the problem of the tv getting confused between arc ultra/amp both being connected and not knowing which to prioritize. Then, I connected the ceiling speakers to the system by adding the amp to the Sonos/sub 4 system by selecting the ‘add surround’ choice in app. That got all the speakers working and talking to each other. Then, used True Play advanced set up and the system balanced the sound with the in ceiling speakers. Final result: balanced Dolby Atmos sound in our space using the arc ultra, sub 4, and in ceiling speakers connected by an amp. Thanks all!

​@Anna B Are you happy with the sound you are getting?  We are building a new room and I would prefer the in ceiling speakers with the Ultra and Sub 4, but I suspect I would get better sound using Era 300s as my rear speakers.

I am happy with the sound, and I do this it is comparable to what we would have achieved with two 300s. Though, I haven’t compared the sound experience directly between the two in my space, so I’m just going off my experience. I used the first 3 minutes of Top Gun Maverick as my unscientific comparison between my set up in my space, and a friends set up with ultra, 300s and sub 4. For me, the sound in both set ups was incredible and, if there was a difference, it was too negligible to notice. That said, I do think ceiling speaker placement is important and you need to have the back speakers just behind your listening spot to achieve the proper downward firing sound that will create that Dolby atmos experience. I have two left and to right speakers (total of 4).  Two are midway down the room and two are just behind the listening area. I believe location is key. It doesn’t offer the flexibility of being able to move the speakers around like I would be able two with eras, but I like the cleaner look of having the speakers, wires and plug tucked away. So, each set up has its pros and cons. Finally, it’s important to use the advanced setting of the auto tuning to balance the speaker set up. Before I did that, the sound was wonky and I thought I’d made a big mistake. But, the tuning working beautifully to balance it all together and achieve a really good sound. 

​@Anna B Thank you for your reply, you have me confused again.  😃  I was resigned to using 300s even though I much prefer the cleaner look of the in ceiling speakers.  But, your report means I’m going to have to rethink my decision.

So are you powering the 2 pairs of speakers with one Amp or 2 Amps?  The ones that are midway down the room, does that mean they are in front of the listener?

You can connect four speakers to a Port, but now ​@Anna B is getting sound meant for rears from in ceiling speakers placed in front of her and behind her, if I understand her set up right. That would sound very strange to me.