I had the same issue. I had to remove the first sub from the room, then I added the second sub first and the first one at the end.

Thank you for the solution! Finally it is working. Cheers

I am unable to remove my first sub! Error message.

This solution worked for me as well when trying to add the second sub to the system. I removed the ‘old’ sub gen2 from the ARC (home theater setting). Then I added the new sub gen3 to the ARC and after that the ‘old’ sub gen 2. Both where added without warnings.

It is not advisable to connect a secondary speaker like a Sub or surrounds with Ethernet unless you also have the main speaker, soundbar or Amp connected first.

That being said, the Sub Gen 3 needs to be the first sub you pair with the soundbar because only the Gen 3 has the ability to have the second sub added to it.

Same issue for me but with 2 sub gen3. I turn around since one hour. I try to remove first sub, always error message, about my network, but my network work perfect.

Very frustrating, I just pay + 900$ for second sub……. ;-(

Uninstall sonos apps, reinstall, same issue, Second sub is connected to my network, but not assign. Impossible to assign…

I finally succeeded, I did a factory reset of the ARC soundbar. Because it is the centerpiece of the home theater, this erased the entire living room, the Arc, the two era 300s and the sub. Then I first connected the Arc, then a gen 3 sub, then the second gen 3 sub, then the two era 300 surrounds.

I just completed the trueplay calibration, and I started the playlist Dolby Atmos on Amazon music unlimited, from the sonos apps, and the difference between one and two sub gen 3 is immediately noticeable. It gives a richer, more complete sound on the two era 300s, yes, I'm amazed.

Large room here, 12 foot cathedral ceiling in the tip, vertical audio at 10, gives dolby atmos sound much better than 0.

Only one control for the 2 sub, not one for each. So I put +4 for sub… oh my god… :-)

Ouff… my god…

Subwoofers are very sensitive to room placement. Walk the room, playing a thumpy bass track, very close to the walls, with a handful of Post-It’s and a magic marker. You’ll discover spots where the bass is accentuated. Add a Post-It here. There will be a few spots where the bass is extra accentuated. Mark these with !!. If you place a subwoofer or listener at one of these “Peaks”, bass will be increased. Midway between Peaks there is a “Null” where bass will be minimized. Peaks and Nulls are more or less evenly distributed around the rooms. After you have some experience finding Peaks and Nulls along the walls, you’ll be able to find smaller Peaks and Nulls (in 3D) throughout the room.

In my college apartment I found a monster peak and placed a chair on it. This was a favorite spot for males because bass seemed to explode from inside your body. If I moved the chair a few inches the peak was gone.

Thanks to share these information. I made some test here. For now, this is my setup at home.

My setup (128mb) video.mov

Thank you for this information, I will do some positioning tests.
 

This is my setup for now.

Lower right corner, sub #1, and the Arc to its right not visible, under the TV. And the second sub, behind the couch.

So I placed the two subs to direct the sound in this angle « \ ».

I am especially surprised by the increase in sound quality of the two Era 300s with the addition of the second sub. 

To the left of the telescope is the dining room, so it's a large open-air space. So the addition of the second sub there, behind the Era 300, acts a bit like a second room from the dining room point of view. It fills the whole space much more evenly than with a single sub.

By the way, I would not use Time as my test track. I prefer a thumpy rock radio station because there will be more “thumps” per minute.

Time, its only one track, just happy last night when all is working good  .
 

I listen mix of dolby atmos, ultra hd, hd, stereo 

What is your favorite config, like equalization, and audio surround, audio sub, audio vertical?

It probably varies depending on what you listen to, style of music and, or, movie?

Only you know what sounds “best” (to you). I know a fellow who always sets bass and treble to max and placed his SUB at one of the “!!” spots that I mentioned above. He loves it. I’m so glad that I only pass through there every few years.