The Beam alone is a 3.0 sound bar. So with 5.1 audio, the Beam has to downmix the audio into a stereo signal while using the left and right speakers to produce virtual surround sound. Adding surround speakers will allow the Beam to focus on playing only the center, front left, and front right channels while leaving the left surround and right surround channels for the surround speakers. So, yes, adding surrounds should improve the audio performance from the Beam.
The Beam alone is a 3.0 sound bar. So with 5.1 audio, the Beam has to downmix the audio into a stereo signal while using the left and right speakers to produce virtual surround sound. Adding surround speakers will allow the Beam to focus on playing only the center, front left, and front right channels while leaving the left surround and right surround channels for the surround speakers. So, yes, adding surrounds should improve the audio performance from the Beam.
Great, thank you. Side note, I’ve seen your posts on other threads and you seem so knowledgeable. Two quick questions:
1- I saw a thread where you recommended LG CX/ C1 OLED users to set up their Beam 2 using eARC and setting Digital Audio to Auto instead of Passthrough - is that still the case? I always thought passthrough should always the preferred choice to avoid extra processing and delays?
2-When I check the Sonos app, it looks like the eARC audio passthrough is coming as 7.1 (not 5.1). Should I set my PS5 to 5.1 ch or 7.1 ch? (given the audio is being sent as 7.1)
1- I saw a thread where you recommended LG CX/ C1 OLED users to set up their Beam 2 using eARC and setting Digital Audio to Auto instead of Passthrough - is that still the case? I always thought passthrough should always the preferred choice to avoid extra processing and delays?
Passthrough is usually recommended. Choose the setting that gives you the best audio quality.
2-When I check the Sonos app, it looks like the eARC audio passthrough is coming as 7.1 (not 5.1). Should I set my PS5 to 5.1 ch or 7.1 ch? (given the audio is being sent as 7.1)
Since the Arc maxes out at five channels, I would choose 5.1.
Just my personal opinion, but I think you’ll get more audio improvement by adding a sub before adding surrounds.
I’m debating adding two One SLs as surrounds to my Sonos Beam 2.
After I add the two surrounds, does this improve the audio of the Sonos Beam 2 by redirecting some of the previous channels the Beam was outputting to the new surrounds and therefore widening the soundscape of the Beam 2?
Or does the Beam 2 continue outputting the same audio as before the surrounds and the surrounds simply get extra sounds not present before?
Just wondering if adding the surrounds further improves the audio quality coming out of the Beam 2 basically.
This is except my situation too. I recently bought my very first Sonos product - Beam 2 and I'm loving it so far. Personally I certainly don't need the sub as it's already providing enough bass for my taste (and my neighbor's lol). Also thinking of I should add 2 Ones or the One SLs.
I got the Beam Gen 2 early Feb, then added the sub then 2 One SLs. I have to say I love this set up but I think I noticed the Ones more than the Sub. The surround sounds are just amazing and well worth getting. The Sub is too but you will be amazed at the improvement in just adding the Ones 
Just my personal opinion, but I think you’ll get more audio improvement by adding a sub before adding surrounds.
The surrounds add a lot more soundstage and immersion. Much cheaper too.
