Just to clarify a bit on what the current capabilities are.  When an Arc+Sub+ 2 surround speakers are playing home theatre as a single entity, they are usually referred to as bonded.  Bonded speakers always play at the same relative volume, and have specific audio content/channels they play for the source.  When bonded, you can’t airplay to a specific speaker, or pair of speakers, in that room and have it act individually from the rest of the speakers.  You can choose to have just the front speakers play for music stereo sources, or have the front and rear speakers play.  With atmos audio, all speakers play.  

If you setup a pair of Era 300s as one room (a bonded pair), and the Arc+ Sub as 2nd room, you can group them together to play the same audio, but this is not the same as have all the speakers bonded together.  Your Era 300s room will play all the audio, as if it’s the only speakers in the room, and your Arc+sub will also play all the audio.  There is no ‘dividing up the work’ in this scenario.   This would not be an “emersive experience”.  It may sound good to you personally, but not objectively what people look for in an atmos setup.

This is a common request.  (A similar request is to be able to easily move a sub between different Sonos rooms without having to rebond and retune every time).  Personally, I’d prefer Sonos work on developing a home theatre setup that includes separate front speakers for the ideal sound for music and TV, rather than what looks to be a half way soltuion of bonding and unbonding speakers quickly depending on the source.  Soundbars have always been a compromise on stereo sound for convenience and simplicity, and it seems like the market is shifting back to interest in separate front speakers.

Please fix this, Sonos. I’m sure it’s a tough engineering problem and maybe requires rethinking some fundamentals. An update would be nice. 

One of the reasons I returned the 300’s…switching back and forth.

It’s important to understand what is at work here.  When adding surrounds, the radios are reconfigured to a one-way dedicated 5 GHz connection from the soundbar to the surrounds.  This requires a series of handshakes to get right, which is why adding surrounds takes a bit of time.  The reverse happens when the surrounds are removed.  So therefore, an “easy toggle” between the two is not possible.  They could place a hot button that does the add surrounds/remove surrounds at the touch of the button, but that is not going to speed up the actual process, and doesn’t save much from the system menu commands we have today.

An alternative would be allowing one to mute the front device and just play out of the surrounds, which would appear to be easier to do, but I defer to the actual engineers on the practicality of the idea, especially with regards to spatial audio.   I’m sure switching from sending just surround info to sending a full Atmos audio stream through the surrounds from the soundbar isn’t trivial, and given hardware limitations of the decoder and the way it is wired to the soundbar, may not even be possible.

Appreciate your thoughtful response. Do you think these limitations are due to Sonos’ architectural decisions or are they broader? I’ve also wondered whether they could just excuse one of the speakers (Arc) and funnel everything thru the Eras - this would be just fine if possible. 

These limitations are due to the physics of WIFI.  The direct one way connection is required for the low latency needed for video sync.  However, that direct connection is not robust enough to go through walls and ceilings (hence why it is only used for Home Theater, which is all in one room), so a normal 2.4 GHz connection with a larger buffer for reliability is used for music sources.  

As to “funneling everything to the Era 300s”, that may be a solution, but it doesn’t necessarily work for Atmos audio, which requires a lot more than the 2 channels to each speaker that Atmos Home Theater does in order to reproduce on the Era 300’s.  It also causes a problem when grouping with other rooms, a core feature of Sonos.  We also don’t know if the hardware in the Sonos soundbars can even separate those channels for transmission to the Era 300’s and not play them itself.