@YoYoPedro 

To be correct in terms the pair of OneSLs has not to be „grouped“ (what they could be if they are set up as a seperate stereo setup room), but they have to be bonded as part of the ht room to the Arc. 
If so, your setup indeed is Dolby Atmos ready. Just with the limitation, that all atmos rear channels (back rears, side rears and hights) are played as a combined signal by the OneSLs. 
Era speakers would do this as separated rear channel signals (in case of Era100 2 of 3 by virtualization). 

Thank you! The wording I had seen made it slightly unclear re: the OneSLs being “grouped” and their effect on “Atmos readiness.” They are not “grouped” as a stereo pair, they are only “grouped” as the rear surround speakers with the Arc and Sub (Gen 3). All the best to you and your family for your prompt and concise answer. Would my music (not TV) listening experience be radically improved if I replaced the OneSLs with Era speakers as surrounds? Or are we talking minor nuances? Thanks!

Perhaps see this ultimate guide to a Sonos Home Theatre system too…

https://www.sonos.com/en-us/blog/the-ultimate-guide-to-a-sonos-home-theater

Thanks, Ken! That helps explain somewhat. That explains the physical differences, but I would love to get feedback as to actual experiences/differences specifically with MUSIC listening differences as opposed to cinema. I suppose it really depends on the recording/mixing. And how much of the music that I listen to is even available as Atmos or Spatial Audio recordings. Cheers!

@YoYoPedro

Yes, of course you are right… technically it’s the same for tv or music and there the atmos signal has the same opportunities to play the available channels depending on the used model of rear speakers. 
But it strongly depends on the mixing. And my personal experiences are that there sadly are not as many atmos music tracks of such a high quality standard. A few are and probably there you can hear separated back and side rear channels with Era300, but imho at the moment it isn’t worth it. 
So I think for most atmos music content you are still fine with a pair of One(SL) as rears. 
If atmos music will break the wall and become some standard, the quality of upmixing hopefully gets better and a pair of Era300 will be woth buying just for that. 

I currently have the Sonos Arc, Era 300s and dual Subs in my living room and use Amazon Music Ultimate and Netflix/Prime Movies and I find it’s great to hear the rain, planes, helicopters etc. overhead, or have it feel like bullets are whizzing around the room, past my head, in some movie action sequences, but these thing can depend on the listening environment and where the listener sits in a room.

I’ve had the Sonos Arc in the past setup with Sonos Ones and even Sonos Fives, but I personally much prefer the Era 300s as my HT surrounds.

I also have a Sonos Ray in my Kitchen/Diner and that has Era 100’s for its surrounds and a Sub Mini. I find they’re very similar to Ones, when used as surrounds, in my humble opinion, but crisp and clear with their audio output… So my own choice is probably clear.

I would personally add Era 300s to an Arc setup, then next I would choose Fives (but that’s a bit of an overkill, as they sound way better as a stereo pair for music audio), then next would be Era-100s and my last choice would be Sonos Ones. 

But that’s just my own order of preference, if that perhaps may help here at all.

@Ken_Griffiths 

I completely agree if someone like you is also or even mainly using the setup to watch movies with atmos audio. But for just listening to atmos music imho it’s not worth because of the reasons said before. There are many movies I really like for their great atmos audio, but just a very few of atmos music tracks that get me impressed. But don’t know exactly… maybe my claims to atmos music are too high. 

I’d just want the best for my Home Theatre setup, as I use it for both movies and music. I would not choose the lesser option because of the music format/type of music I listen too… in fact I still like albums tracks that are stereo only, but my choice of speaker would still be the Era 300s because I would want them ready for movie audio anyway.

In fact the Fives I mentioned are ‘out in front’ in the same room as my Arc HT, as I prefer those (set vertical) for music playback. I much prefer the soundstage out in front of my listening position, rather than to the sides, or behind the sweet-spot. I sometimes put one of the Subs over to that stereo pair too.

@Ken_Griffiths 

Congrats… imho the perfect Sonos Setup, if you live in both worlds… movies and music.

I would like to expand my system to that level too, but there are some limitations for me such as room situation (especially for bigger rearspeakers), overall room design and of course the waf.

I really hope Sonos will come around with an amp in future that supports eARC including Atmos and maybe even the option to add something like an Era100 wireless as a real center speaker. 
That could be my all in one setup… just dreaming.