Yes, you can use the Ones as surrounds with the Arc or Beam. But you should be able to add the Ones as surrounds to the Amp. What exactly happens when you try to add the Ones as surrounds to the Amp?

You cannot use a Sonos sound bar and the Amp together as front speakers in a Sonos home theater setup. You can only connect the Amp as surrounds to the Arc or Beam. You can group the Amp with the Arc or Beam, but you will experience a slight audio delay from the speakers connected to the Amp when listening to TV audio. Streaming music will be in sync though.

Hi @sonos.keeps.getting.worse 

Thanks for your post!

As @GuitarSuperstar mentioned, you absolutely can bond two Sonos Ones as surrounds to an Amp. The most common reason for being denied this in the app is if the two Ones are already bonded as a stereo pair. Unbond them and put them in some temporary rooms (the ones the app suggests will be fine), then go back to the Amp’s settings and Add Surrounds. 

If this fails, please try from another phone/tablet, or reboot your router and then Sonos, once the WiFi is back.

I hope this helps.

Thank you, that was indeed the issue, I had them as a stereo pair.

Can you explain what you mean by this:

You can only connect the Amp as surrounds to the Arc or Beam

Does that mean the spatial audio won’t work as well, because they’ll be treated as side speakers even though they’re physically in the front?

No, he means you cannot configure a Beam/Arc as a front center and use the Amp for the front left and right.  An Amp can only be used as surround speakers when using a Beam/Arc as the front L/C/R.  Or you can use an Amp as front L/R (phantom center) and either another Amp or 2 Ones/Fives as surrounds. 

So I guess my main question is, is there any downside to having the Beam/Arc + 2 wall speakers in front + 2 Ones in back? Besides having more speakers than necessary?

There is no benefit to TV sources.  Matter of fact, if you grouped them with the Beam/Arc+surrounds, TV sources will have a distinct echo.  If you wish to just use the Amp+fronts for music sources, then the quality of those speakers will more than likely be better than the Beam/Arc

Well this is all irrelevant, because after talking to Sonos support and sharing my phone screen with them, it turns out the stereo pair was not the issue - my Connect:Amp is simply too old to support setting up a surround system.

So my username remains accurate and yet again I’m left with a $600 brick just like my old Play 5. Thanks Sonos

Huh? The CONNECT:AMP doesn’t have a 5Ghz antenna in it to wirelessly connect to a front soundbar, but it can be used to power surround speakers, as detailed in the setting up Amp or CONNECT:AMP as surround speakers FAQ.

The Connect:Amp powers my 3rd party wall speakers but I can’t add my two Sonos Ones as surround speakers. The app doesn’t even give me the option to “setup surrounds” or any of the “home theater” or “TV setup” options.

I mean how f**kin short sighted of a company do you have to be to make an Amp that can’t connect to your other speakers for a surround setup?

Exactly how are you supposed to use the Connect:Amp for surround if it has no HDMI-ARC or optical input?  The Connect:Amp hardware has analog stereo input only, which means surround sound is impossible, the hardware just isn’t there.

Given when it was designed, and the purpose for which it was designed, manufactured, marketed, and sold, it’s unsurprising that the Sonos CONNECT:AMP doesn’t handle the front side of a home theater. I’m fairly impressed with the ability of Sonos to give it the capability to handle rear surrounds. 
 

Clearly, the marketplace had moved from when the CONNECT:AMP was designed, so that when the replacement device, the Sonos Amp was released, it had both the electronics and port necessary to receive and decode a Home Theater type of signal, as well as the 5Ghz ability to bond with surround speakers. But there were many years between the design of each product.