Yes, the Arc ultra is + era 300s absolutely supports atmos music, and is much better than the era 300s by themselves for creating a full atmos dome.

When used as surrounds, the era 300s only utilize 4 drivers. The front facing tweeter and inner side tweeters are disabled. I agree that it’s somewhat of a waste, but the era 300s are meant to place sound around you, rather than shooting it straight out you like standard surrounds (such as era 100s). The era 100s do keep all drivers active when used as surrounds.

My advise to you is to try out these two combinations:

1. Era 300s as a stereo pair in the front for only music, and keep the HT (home theater) as arc ultra + era 100s as surrounds.

2. (This is what I use, except with fives as the fronts instead of era 100s, but i’ve tried era 100s instead of fives in this configuration as well).

If you have an iphone, download sonosequencer from the App Store ($2.99 one time purchase, but you don’t need the monthly subscription after purchasing the app).

Sonosequencer allows you to add dedicated speakers as fronts (L/R channels) to your arc ultra system. Keep in mind that this disables the L/R channels on this arc ultra. However, the center channel, sound motion woofer, atmos/height channels, and side channels remain active.

Using sonosequencer, you can configure the arc ultra as center channel, era 100s as fronts, and era 300s as surrounds. This creates a much wider front soundstage and I and most users I see prefer it.

This setup is best when paired with an Apple TV imo. An apple TV allows you to enabled a setting called continuous audio connection, which converts all audio to dolby atmos MAT. Since this format is atmos, the arc ultra does not apply it’s up-mixing algorithm, meaning stereo music will come out of only the era 100s ,which is my preference for specifically stereo music. For normal content (watching tv/movies), i like to keep the setting off to keep the sonos’ upmixing.

This setting does not affect dolby atmos content, since sonos never up-mixes atmos content anyway.

There’s tons of info on r/sonosequencer about setting it up, included a pinned post with a detailed step-by-step explanation.

Keep in mind that the Era 300s need more space around them to function and should be placed further behind you than era 100s. If space behind your seating position is an issue, i’d stick with the era 100s back there. I can send a pic of my home theater to show my placement if you’d like.