**Big WOW Factor** The Gold Standard with a Few Handcuffs

 The biggest gain you can get by using two subwoofers is the evening out of room modes.  A single subwoofer can suffer from resonant frequencies bouncing off the ceiling & walls causing peaks and nulls.  A second sub may also suffer from room modes, but because it’s in a different location in the room the peaks & nulls will effect different frequencies than the first subwoofer.  This helps flatten the frequency response curve.  
 If you TruePlay the room with the two subwoofers you should not necessarily get more bass.  I would certainly not want that result, but with two subwoofers you should get better balanced bass.

The reverse situation is also a possibility, two Subs in the wrong locations can reinforce the resonance peaks. 

Careful testing, best with test signals/media/streams,  and trying multiple locations in search of the most even sound at your listening positions is important. It also gives Trueplay the best chance of dialing in the room corrections.

@MoPac

I completely agree, MoPac. My interest in a second sub is precisely to "fill the room" and eliminate those null spots. Currently, with just one sub in the front right corner, I can tell the bass is localized. By adding a second unit… likely behind or under the sofa … I am hoping to achieve that "even" pressure where the deep rumble feels like it is coming from the atmosphere of the room rather than a specific box. It is the "balanced bass" you mentioned that I am after, rather than just raw volume.

@Stanley_4

You raise a very valid point, Stanley. I am aware that simply adding a second sub doesn't automatically mean better sound; if the two subs are poorly positioned, they can indeed cancel each other out or create unpleasant resonance peaks.

I intend to be quite meticulous with my testing. I plan to use various Atmos test signals and my favourite "reference" media to find the sweet spot. My goal is to achieve that deep, rumbling physical sensation by placing the second sub under the settee, but I will be careful to ensure it doesn't "muddy" the room's overall response. As you suggested, I’ll be running Trueplay after every major move to give the system the best chance of correcting the room's unique acoustics. I want that "thump" in the chest, but not at the expense of the clarity I've worked so hard to achieve with the Arc Ultra.

 I’m surprised you detect the location of the sub.  I figured with the Arc Ultra being capable of a frequency response lower than the Arc the crossover would be low enough to make the sub essentially disappear.  I have the Arc & Sub 3 and after TruePlay the sub is not easy to locate, at least with my eyes closed.

 Mine is located on the right side next to the TV cabinet, so not in a corner.  Corners tend to exaggerate bass.

 I was getting a buzzing sound when I first set up the sub.  Turned out to be the shelves of the TV cabinet vibrating against the cabinet wall.  Wedge shaped rubber door stops fixed that.

That is a very fair point, @MoPac. My "localisation" might be slightly exacerbated by my current setup; with a 15-month-old "mischief" in the house who enjoys using the sub as a garage for his toy cars, I’ve had to hide my current unit under a table!

My main concern with adding a second sub behind or under the sofa is that if it's too close, that "boom" might overwhelm the Arc Ultra. However, I’m hoping that by utilising the space behind my sofa, where I already have my Era 300s on stands, I can find a balance that adds that physical "4D" rumble without drowning out the dialogue. It’s a bit of a delicate dance between deep bass and vocal clarity, especially with a screen as monstrous as mine!

As for the 25% discount email, I had the same experience. Once I crunched the numbers, the "deal" didn't feel quite sweet enough for the Sub 4, so I think I’ll stick to my plan of hunting for a bargain Gen 3 on the secondary market.

Have you looked at Soundskins to protect your Sub and be able to put it in a better place?

There are a variety of covers for the Sub to child/pet/bug proof the openings.

https://www.amazon.com/s?k=Sonos+sub+cover&crid=2YL62PEA3Z2RC&sprefix=sonos+sub+cover%2Caps%2C265&ref=nb_sb_noss_1

@Stanley_4

I appreciate the suggestions, gents. I actually hadn't realised there was such a market for Sub covers!

I had considered using the original white mesh sleeve that the Sub was packed in, mainly because I want to keep the "white set" aesthetic without a thick "curtain" blocking the sound waves. However, my main concern with that is heat; I wouldn't want the internal components to get too toasty during a long film if the air can't circulate properly.

I'll certainly look into the white version. it sounds like it might be exactly what I need to keep the system looking sharp while protecting it from toy car incursions!

 I think with class D amps there is no heat issue.  Of course you should research this prior to investing in a cover.