Previous setup:
Arc + Sub (Gen 3) + Play:5s
 

Room dimensions and layout:
20’ long x 13’ wide with an 8’ tall ceiling

The Arc is located on the 13’ wall on a 2’ tall media console. The surround speakers are placed on speaker stands in the corners of the opposite wall on each side of the sofa angled towards my main listening position. My main listening position is a giant Jaxx Saxx (bean bag) in the center of the room about 8’–9’ from the Arc and 8’–9’ from the surround speakers. I can hear the height channel audio from the Arc loud and clear from this listening position.
 

What I watch/listen to:
I listen to A LOT of music in Dolby Atmos and 5.1 on both physical media (Blu-ray, DVD-A) and streaming (Apple TV 4K). I also watch a lot of films in Dolby Atmos and 5.1 on Blu-ray and streaming. I own multiple Dolby Atmos and DTS demo discs on Blu-ray to test with.
 

Play:5s as surrounds:
I have been using a pair of Play:5s as surrounds with my Arc + Sub setup for the last 8 months and have loved it. Many people say the Play:5s (or Fives) as surrounds are overkill, but I completely disagree. The Play:5s are a perfect match for the Arc. When I used a pair of Play:1s as surrounds, the room always sounded front heavy because the Arc’s soundstage sounded so much wider and taller, while the Play:1s sounded narrow and small. The Play:5s brought much more balance to the room and clarity to the rear soundstage. They especially sound impressive when listening to music in 5.1 or Dolby Atmos.
 

Era 300s as surrounds:
Overall, the 300s are an improvement over the Play:5s especially with Dolby Atmos audio for film/TV. The addition of the two height channels and side channels add much more detail to the rear soundstage that is missing from the Play:5s. The 300s do a really good job placing objects at a specific location behind you too. They help create that bubble of sound that you want when listening to Dolby Atmos audio.
 

Disabled front-facing drivers:
The main issue I have with the 300s is the front-facing driver is disabled when it is being used as a surround speaker. This means it relies A LOT on reflecting sound off of the walls. If your room isn’t set up just right, you can lose a lot of detail and volume from the surround audio. This seems to be most noticeable when listening to music in 5.1. When I listen to the 5.1 audio mix of “Women in Chains” from Tears for Fears on Blu-ray, some of the sound effects from the surrounds are much more muted on the 300s compared to the Play:5s. This really takes away from the immersive experience when listening to 5.1 music.
 

Extra channels/drivers, more detail and separation:
But at the same time, because of the addition of the extra channels in a 7.1.4 setup, you gain some detail and separation from Dolby Atmos music that you don’t hear with a 5.1.2 setup. When I listen to “Take On Me” by a-ha in Dolby Atmos, the background vocals are much less noticeable on the 300s than the Play:5s. They sound more diffused than clear and direct. But at the same time, I hear a lot of new detail and added separation from other elements of the song that I don’t hear from the Play:5s. On “Rocket Man” by Elton John, the background vocals from the Play:5s are loud and clear perfectly complimenting Elton’s voice from the front channels on the Arc. But with the Era 300s, the background vocals are more faint and become more like ambient noise rather than a vital part of the song. But I am also able to hear new details from “Rocket Man” from the surrounds that I never heard on the Play:5s.
 

Inconsistent surround/height audio levels:
One other issue I have noticed is the inconsistent volume levels from the Era 300s. Just when I think I have the Surround Audio and Height Audio levels balanced just right with the Arc, I play a song in Dolby Atmos that sounds great and then all of the sudden, a very specific instrument, vocal, or sound effect from the 300s are very overpowering. I don’t know if it’s the specific audio mix of the song or the way the sound is reflecting off my walls and ceiling. But this sudden volume change from specific elements of the song is extremely distracting and just ruins the experience. This doesn’t happen with every song, but I have experienced this more than a few times. I NEVER experience this inconsistency with the Play:5s.
 

Movies:
I have only watched one film all the way through with the Era 300s. It was La La Land in Dolby Atmos. I know it’s not the best film to test, but it was the only movie I could convince my wife to watch with me this weekend. Overall, it sounded great. Dialog was clear and the music from the film sounded very balanced between the Arc and 300s. The highlight of the film was actually the Dolby Atmos “Unfold” trailer before the film started. It sounded MUCH better with the Era 300s.

I was also able to test some Dolby Atmos demos from a demo disc on Blu-ray. With the Era 300s, the “747 Takeoff” demo sounded much more realistic as the 747 takes off from the front of the room and flies overhead to the back. On the “Helicopter Demo”, I could clearly hear the helicopter fly around the room to each of the four height channels. With the Play:5s, the helicopter would just fly from side to side from the two height channels above me. The raindrops and thunder from the “Rainstorm” demo felt even more realistic than ever before. So with these demos, the Era 300s are a pretty big improvement over the Play:5s which leads me to conclude that movies in Dolby Atmos is where the Era 300s will truly shine.
 

More testing needed and initial impression:
I still need to do a lot more testing before I make a final decision especially with films and music in 5.1 and 7.1. Right now, I am leaning towards keeping the Era 300s as surrounds. They aren’t perfect, but still an improvement over the Play:5s. Even though I am missing some detail from the surrounds when listening to music, the added detail and separation from the extra channels more than make up for it.

I’m not sure if Sonos will do anything different about the front-facing drivers on the 300s, but hopefully they will figure out a good way to utilize those drivers without compromising the sound of the rear soundstage. If they are able to bring out some of those lost or diffused surround elements I mentioned above using the front-facing drivers, the Era 300s will be just about perfect IMO.

And like all Sonos speakers, I am confident that Sonos will gradually improve the Era 300s over time. The Arc sounds 10x better today than how it sounded when it was released in 2020.
 

One more thing:
When I first set up my Era 300s as surrounds, I angled them towards my listening position and ran Trueplay. They sounded HORRIBLE. They sounded so bad that I was already considering returning them. But after I straightened the speakers so they faced the opposite wall directly and redid Trueplay tuning, this made all of the difference in the world.

I hope this is helpful. Feel free to ask any questions.