I definitely would not recommend the Arc nor Beam Gen2 as music is your primary objective. IMO the Arc or Beam Gen2 would add unwanted color to your music. Here are some options:

1. Sonos Ray for TV assuming it has an optical connection *. You’ll get basic Dolby Digital 5.1 and DTS sound (Left/Center/Right Channels) for movies. Add an Sonos Sub Mini and a pair of Era 100’s as surrounds **. When playing music all speakers can play at full volume placing you in the center of the sound stage.
2. Sonos Ray for TV only.
3. A pair of Fives or Era 100’s with Sub Mini for music that produces a traditional Left/Right stereo sound stage. The Era 100’s when not used as surrounds also allow you to send music to them via Bluetooth. You could group the Ray to the Five’s or Era 100’s to have it play along as well. You can send music to the Ray only; if you desire. Note: The sub is not active when music is only sent to the Ray; as it is bonded to the stereo pair.

 * If TV has no optical connection you’ll have to consider the Arc or Beam Gen2. I would suggest the Beam Gen2.

 ** If you want bigger bass then consider the Sub Gen3

TV has HDMI ARC

I like the first option although might splurge on Beam unless that’s a bad idea.

Room is rectangle with TV facing diagonally in corner.  Sofa along long wall opposite TV and another in L configuration to Sofa 1 - also facing TV.

Best placement of speakers?

If you want to splurge on the Beam Gen 2 that’s fine. The upside is you’ll be able to use the Audio Return Channel (ARC) feature with Alexa or Google voice assistants. You’ll need eARC to experience Dolby Atmos with the Beam. There is a 3rd party device by HDFury that will allow Dolby Atmos audio on the Beam in the absence of an eARC HDMI port.

If you use the Ray (or Beam) as a TV speaker only I’d place the Five’s or Era 100’s left and right of the TV. You may have to experiment with placement of the sub…typically in a corner to start. 

If you use the Fives or Era 100’s as surrounds the placement should be left and right of the viewing/listening position about 2 feet to the rear (ideally). Again you may have to experiment with sub placement. 

Great answers!!

Also, would there be a significant upgrade in sound quality with 2 300’s rather than 2 100’s?  

I have Spotify so at the moment not much if any Atmos support

The Beam can process DD+, so as I understand it Atmos from most TV apps should not have to be a problem even without eARC. Uncompressed Atmos (like a BD player or Apple TV can provide) needs eARC. Compressed Atmos does not.

Note, however, that while the Beam can process DD+, it does not have the dedicated upward firing speakers that the Arc does. However, that wouldn’t affect the rear speakers, indeed 300s would provide better surrounds, especially when using DD+. 

Sorry for the late response. The question you are asking becomes a little difficult to answer and here’s why…

When it comes to listening to music I’m a purist. I want discrete left and right channels for stereo separation. IMO sound bars of any brand add coloring to music. They infuse a bit of electronic trickery to mimic discrete Left/Center/Right channels as found in a traditional home theater with individual speakers for the aforementioned channels. That same trickery IMO has an impact on a stereo recording.

The Sonos Arc, Beam and Ray do what they are designed to do and deliver Left/Center/Right channels for home theater. When playing music there is a certain amount of bleed through on the center channel WIMO (at least to my ear) changes the dynamic of a stereo recording. I still listen to Vinyl (records) which when recorded only have (or had) left/right channels to create the stereo sound stage. 

Sonos sound bars when used with surrounds allows music to play through all speakers as I mentioned earlier. That places the listener in the center of the music instead of a traditional listening position. Some love the effect. I for for one do not.

Now let’s answer your question…here again IMO...

For the best Sonos music listening experience I recommend two Fives over two Era 300’s. The Era 300’s are excellent speakers but (IMO) lack a bit of the low-end (if not paired with a sub) that the Five’s deliver without a sub. Of course adding a sub to the Five’s makes for an even better listening experience.

There are Rumors of Era 500’s on the horizon that supposedly will remove the gap in the low-end found in the Era 300’s vs the Five. However, that’s all rumor and I don’t see Sonos releasing an Era 500 in the very near future. 

In case you’re wondering I love movies and music. I have an Arc with Sub (gen3) x 2 and Era 300’s as surrounds. However, I have a set of Fives (in the same room) for music.

My preferences aside here’s my recommendation for you that IMO checks all your boxes...

• Beam (Gen2) 
• Era 300’s 
• Sub (Gen3) or Sub Mini

Configuration #1 *

1. Setup the components as a Home Theater and listen to music. 

Hopefully you can setup a Spotify playlist. After a thorough listening session with various types of music...remove the Era 300’s and Sub from the Beam

Configuration #2 *

• Setup Era 300’s with Sub and listen to the same Spotify playlist.

Configuration #3

• Send music to the Era 300’s via Bluetooth **

Configuration #4

• Using Configuration #2 and later Configuration #3; after starting the music Group in the Beam

After the doing the above decide which setup you prefer. Remember that if you choose Configuration #2 the sub is not available for TV viewing.

* Be sure to listen/view movies in both configurations. You may be surprised at the difference in how the Beam handles that experience which in the end may be the deciding factor.

 ** Bluetooth for the Era 300’s is not available when used as surrounds

Again, great answer - exactly what I was looking for.  Just want to make sure that I would not be losing sound quality by going Bluetooth rather than Wi Fi regarding signal transmission.

Your speakers will always be on WiFi.  Bluetooth is just a feature/option. Prior to the Era Series the only speakers with Bluetooth were the Move and Roam both of which are Portables for outdoor use. People have asked Sonos to incorporate BT in its non-portable speakers for years. Now they have it.

One use scenario for Bluetooth would be to allow a guest to stream to the Era 300’s from their personal device. Any other use for BT would be up to you. 

I also wanted to emphasize that the BT feature is not active when the Era 300’s are used as surrounds.

Last…In theory BT will offer less fidelity than WiFi. However, only you can judge that by listening. If I wasn’t clear…BT does not replace WiFi as the primary means under which Sonos speakers operate.