The biggest difference from adding a pair of actual surrounds is that the “virtual” surround the Arc had been providing now becomes actual 5.1.2 surround (when paired with a Sub and a compatible set of surround speakers). Due to the way my living room is laid out (opens in the “back” to the kitchen and dining room), the Arc couldn’t do anything at all to provide a sense of virtual surround as there just wasn’t any walls back there for the sound to bounce off. 

Adding a pair of One SLs as true surrounds made all the difference in the world. Not only did the Arc maintain the incredible front soundstage it already had, but now discrete surrounds envelop me in sound and dramatically widen the overall soundstage and significantly enhance the experience. 

I want to say I read somewhere that the Arc redirects the drivers it would otherwise have been using for virtual surround to other uses (more bass?) but I can’t find where I read that. And, indeed, I have no idea how the addition of a Sub affects that. 

All I can say is that the combination of an Arc + Sub + One SLs has proven to be an immersive experience with a super wide soundstage, awesome bass, great height effects (for Atmos - if you’re sitting in the right spot), and just an overall amazing sound coming from “just” a soundbar. 

The above is my understanding as well.  When surround speakers are present, I believe the side speakers are redirected to play the left and right front audio to help give the sense of a wider sound stage.  I don’t think they are used for additional bass when a sub is present.  And I agree that if the space and budget allow, it’s a good idea to add the surround speakers.

FWIW, I wrote a review over on Reddit that goes into excruciating detail about my thoughts of Arc vs Arc + Surrounds vs Arc + Surrounds + Sub if you feel like wasting 20 minutes of your life and  want a deep dive into my feelings on the matter. ;-) 

https://www.reddit.com/r/sonos/comments/he1lv9/arc_one_sl_sub_review_from_firsttime_sonos_owner/

Thank you!, that’s what i need it!, was a nice reading,, but i wish to know what sound test did you use? i hace a tall ceiling and im afraid that i dont have much sound bouncing into the ceiling, making the test you did maybe it help me to find the sweet spot!, again thank you for the nice reading.

Happy to help! 

I found the Atmos 5.1.2 test tones here: 

https://www.reddit.com/r/sonos/comments/hc21uu/dolby_atmos_512_test_video_file_download_link/?utm_source=share&utm_medium=web2x

And you can download the official Dolby Leaf and Amaze Atmos trailers from their site: 

https://www.dolby.com/about/support/guide/dolby-atmos-trailers/

There are a LOT of other files of this sort available over at Demolandia as well: https://www.demolandia.net

Have fun, and enjoy!