Welcome.
 

A great thing about Sonos kit is its flexibility. Try the ultra alone for a while. Then but the eras and add them to heat the differences. If you like it: keep them. Don’t like them: set them up in a different room. Or return them within the (30-day?) returns period for a refund. 

Hi Nik,
Thanks for the welcome 🙂.
Your reasoning is certainly correct, but I was interested to hear from someone familiar with Sonos products whether the Arc with the Era 300s is a balanced system overall; and also whether a Sub 4, based on your experience, can easily fit into a small room (perhaps by lowering the sub's level via the app).
If I understand correctly, with the addition of the Sub 4, the Arc Ultra's frequency response is cut at the bottom, and this is another variable that affects the system's output.
As you wrote, I could try and possibly return it, but I'd get the components from a friend/seller who wouldn't be able to take them back once ordered.
Of course, nothing stops me from purchasing the Arc from him and the rest of the parts online...

Do you have a drawing/picture of the room?

An Ultra will give a much better experience than sound from the TV though.
Would definitely go for the surround Era 300 even though the Ultra is a fantastic soundbar.
Fortunately, the Ultra has quite a lot of bass, but would definitely recommend a Sub 4 as well. It just gives a much deeper bass and experience.

The Sub 4 can be placed pretty much anywhere, but then there would be a difference in the bass depending on where it is placed. It is correct that the Ultra scales down the bass when a Sub is connected, that is natural since the Ultra can in no way cause the big bangs/shakes that a Sub can, it does try and do it ok.

Regarding your room, I would consider putting some acoustic panels up on the wall away from the TV or maybe on the ceiling. They are quite cheap and super easy to install.

The sub will complete the Arc Ultra. It allows the Arc Ultra to focus on frequencies other than the low-end. The rears are great as well to afford a full immersion experience. For the record I have three such setups. That said I’m very familiar with Sonos and your proposed setup 😀

However 13 square meters doesn’t say much as to the length and width of the room. Therefore as has been suggested either post a diagram of the room or at least provide length and width dimensions.

The room is 3.25 meters wide, 4.20 meters deep, and 3.10 meters high. There are curtains and a large fabric sofa, a 2.80 x 2.40 rug, and a table in the center. The room has no echo and has decent acoustics. I certainly can't acoustically treat it with panels because it's both my TV room and my dining room. I already have a 28-square-meter room dedicated to music, where I have a large 2-channel sound system; so the TV room needs to feel as much like a normal room as possible. The 77-inch Panasonic Z95B, which I've had for a few days, is already quite intrusive.By the way, to answer Peter, this TV has a built-in sound system that's quite different from the rest of the competition. It has a sort of front-facing soundbar, speakers on the sides, above, and a small subwoofer at the back, but even so, I'm not entirely satisfied: the sound feels a bit boxed and nasal, with dialogue sometimes unclear. That's why I thought I'd try a Sonos system.

Get the Sub, put it behind the couch with the 300s.

You won't regret getting it based on my listening here. The Ultra is pretty good but adding the Sub is a big step up. It is easy to toggle on and off as you are listening and see and feel the difference.

I have two Subs in a 10x10 foot room, one on a Beam and the other on a pair of 300s. It is not too much speaker for the space although a mini-Sub might have done decently there. Didn't try one as I had the Subs available. You might try a mini but your space and the Ultra make the full sized Sub a likely better fit.

You can return it if you are not satisfied but that would surprise me.

You are correct it is a small room. The upside is that you have the opportunity to buy the components and experiment with different setups using the Arc Ultra as the anchor. Then return the components you feel you don’t need  (sub or surrounds or both). Only you can determine what configuration sounds best to your ears. In a room the size you have you might consider a Sub Mini. 

One more thing:When I started this thread, I hadn't read any other topics here on the forum. Then I looked around and came across a thread accusing the Era 300s of not working well when used as surround speakers. I saw a long discussion ensuing (which was then interrupted without understanding the outcome), where the tone among forum members even got a little heated.This alarmed me a little, and I'd like to know if, after about a year, all these limitations (real or perceived) are still cause for controversy or if everything has been resolved and explained.I'm asking because I haven't purchased yet and would like to be sure before doing so. Otherwise, I'd go for just the Arc Ultra, without the Era 300.

Thanks again.

If you believe the Sonos representative’s answer, then it is resolved.  If you don’t believe Sonos and think every driver of every speaker must always be utilized in every possible configuration/room, regardless of sound quality and/or sound engineer intent, then it isn’t resolved. 

One more thing:When I started this thread, I hadn't read any other topics here on the forum. Then I looked around and came across a thread accusing the Era 300s of not working well when used as surround speakers. I saw a long discussion ensuing (which was then interrupted without understanding the outcome), where the tone among forum members even got a little heated.This alarmed me a little, and I'd like to know if, after about a year, all these limitations (real or perceived) are still cause for controversy or if everything has been resolved and explained.I'm asking because I haven't purchased yet and would like to be sure before doing so. Otherwise, I'd go for just the Arc Ultra, without the Era 300.

Thanks again.

Read the answer by ​@Jamie A in the post below. Ignore all the other comments 

The Era 300’s sound excellent. There were aspects that Sonos made adjustments for based upon comments in the community. However, At this stage since their introduction I’d say the sound parameters are set. There are always going to be people who think they know better than the designers. My advice is to ignore the noise. 

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.