Arc Ultra + Era 300 (without Sub 4)

The concept that, in the surround configuration, not all drivers are operational, as this would compromise the proper functioning (as intended during production) of the rear channels, is clear to me.

Reading some posts, it seems that only the center tweeter aimed toward the heads is disabled, but other users suggest that more than one driver is "muted."
In the end, how many of the six speakers mounted in the Era 300 are operational?
This is just for information and to understand how it works, not to make a purchasing decision based on the number of speakers.

I forgot: I hope the distance isn't a problem, the two Era 300s would be placed close to the sofa, so quite close to the ears, if I understand correctly there is a setting in the app to select the correct distance.

Trueplay would take care of any fine tuning due to distance.  I don’t own ERA 300s for rears, but my ones are on either side of the couch and at ear level.  One is closer to my seating position than the other, and after Trueplay, I find they are perfectly balanced.  

As to the drivers being muted, each room is different.  Anecdotal complaints about one or more drivers not being utilized may or may not be the case with your particular room.  As ​@AjTrek1 said, much of it is noise, especially the “I paid for the whole speaker!” complaints that ignore all facts about modern spatial codecs.

I'm happy enough with mine I'm not interested in trying test tracks but if you are the Dolby site has some and explanations as well so it would be worth your time if you care about that sort of thing.

Finally, I've carefully considered the placement in the room, and I'm thinking of ordering the complete package (Arc Ultra + Era 300 + Sub4). However, I want to be sure of this: if I decide to turn off the Sub4 at any time via the app (the option shown in the photo), will the Arc Ultra simultaneously return to full low-end output, seeing as the subwoofer is no longer present? I wouldn't want it to sound cut off at the bottom because the configuration is set to a sub.This is important to me because even though I want the complete system, I'll often find myself listening with just the sound bar, for example while watching the news during dinner. I'm complicated, I admit…😄

Why would you not want the Sub on during the news? Since these kinds of programs are not vary bass heavy, the Sub will not do much anyway, so why would you be bothered?

With Trueplay on the crossover frequency cannot be changed, but you can change the volume of the Sub. I suspect this will not change the low end output of the Arc Ultra - it will still expect a Sub to take care of the lower frequencies.

Great acquisition! You won’t regret it. I’ve similar setup and sounds great, for music or movies. 
In my case, I upgrade with Era 300 in a second step, and the improvement is amazing. I don’t care if they are using all the drivers because the result is excellent, and I know I’ve two excellent speakers if someday I decide to move them alone as a stereo pair. 

Trying to answer to your last question, I’m not sure if when you turn off the Sub, the Arc’s behaviour is the same as if you ungroup the Sub. When I did it is very noticeable, but I have the previous Arc, so I should perform a wider test. Anyway, there is an option “Night sound” that you can active always and it reduces the bass and the volume effects to play the TV in a quiet way. Perhaps it’s what you’re looking for?

At the beginning, when I get the Arc+Sub I thought like you, why to listen the news with all the power of the setup?, but then, you discover that everything is better with the full power, even the news 😅

Ok, I understand that I can leave all the speakers on without any problems.

But with two-channel stereo audio, such as a news broadcast, do the two Era 300s remain completely silent, or does Sonos system activate a simulation to make them emit some kind of sound?

Good question, I recently dive in some forums looking for the answer to that. 
Usually, they are silent, or at least I can’t notice them. But sometimes, with a music or the show tune, you feel them, but after check, the source is 2.0. It seems that Sonos sw does a kind of job and when it detects that the source is feasible to convert to 5.1 it does it. 

I’m sure other users could give you a more detailed answer, but in summary, the rear speakers sound very low always, and sometimes do a louder job. But normally you can’t feel it unless you put your ear on the speaker.

In a live situation there could be hundreds of sound sources within the immediate area. Capturing and reproducing this environment is a very complex task. Sound sources come in various sizes, from a bass drum or battleship to a tiny insect, and from various directions as a result of motion and echoes. Mostly we humans detect sound with two tiny membranes, but there is also some full body involvement (think THUNDER and BOOM) that even a deaf person can perceive. Body involvement goes beyond the ears by detecting gross pressure changes from a BOOM. Because of the human head shape, displacement of our two ears, and the pinnae, our perception, or not, of a sound varies by its position relative to our head. It’s not always obvious as it is with pets and wild animals, but humans subtly move our heads in order to locate the origin of sounds.

You can explore your sensitivity to direction by playing a few single frequency signals of different frequencies on a cellphone, keeping the phone a constant distance from the center of your head. Move the phone laterally and vertically while playing each tone.

In a home audio reproduction system the result seems more realistic if audio can come from various directions, similar to the original scene. In addition to direction challenges there are also reproduction physical and electronic challenges. The large physical membrane required for the BOOM is not well suited for reproducing the sound of a mosquito. Nor can a small tweeter used for the mosquito contribute much to the BOOM. In the electronics we usually think of them, especially the “high end” variety, as being “neutral” and contributing nothing, positive or negative, to the result. I wish this was the case. “Intermodulation distortion” can be significant in power amplifiers. Basically, as the input signal becomes more complex, the power amplifier jumbles things.

OK, after the above digression, there are two basic values with respect to adding a subwoofer. With respect to the physics, the large membrane obviously does a better job of reproducing the BOOM and the smaller speakers don’t need to ineptly attempt to deal with it. With respect to the electronics, removing the extreme bass and routing it to a separate power amplifier reduces intermodulation distortion (humans are very sensitive to this) in the mid and high frequencies — resulting in cleaner sounding midrange and highs. In SONOS systems each speaker has its own power amplifier operating within a custom frequency range.

With respect to removing the subwoofer during news broadcasts or while playing low audio quality media, it’s a lot of bother with little benefit other than saving some power.

Many new to surround users expect to be constantly slapped in the back of their head by surround — else they didn’t get their money’s worth. A good first film to watch would be an action film, such as Top Gun. For these new users it is best to initially elevate the surround level.