If you were to forget about the Arc, you can have a Sonos HT surround system using two Sonos Amps, where the one Amp will handle the three front channels (incl. a phantom Center) and you can attach a 3rd party ‘active’ Sub to that. Then use the 2nd Amp for the rear channels (ceiling speakers). That will work fine for both TV/Music audio. But…

If you were to setup the Arc & Amp for your HT surround system and then also use another Amp with a 3rd party Sub attached (as suggested in your post) - you can ‘group’ that with the Arc and whilst that will be fine for music audio playback, it will ‘highly likely’ be problematic to you for TV surround audio, as the ‘grouping’ process uses audio buffering (approx. 30-70ms delay), which means the Amp/Sub audio will play out of sync with the Arc TV audio. So I would not recommend this approach - you would be much better off selling your 3rd-party Sub and getting a Sonos Sub instead IMHO. 

Hope that assists.

Thanks!  Third party subs are built into the walls, so I think my solution is to sell my Arcs and go with passive soundbars.  Such a bummer!

I have a new TV and installed the Arc. I just bought the Amp hoping to power both two passive in-celing speakers for surround sound as well as an existing third party sub-woofer I already owned, as yourAmp Suer Guid  info says that  is posisble (which I have pasted below.

In doing the installation and getting nothing through the sub-woofer, I discovered - after having to call your Customer Service help line duing the installation - that if using Arc and celing speakers in surround sound mode, you actually CANNOT use a third party sub-woofer, at least not with the latest (gen 2?) of the Amp.

IS THIS RIGHT? WHY IS THERE A SUB OUTPUT ON THE BACK OF THE AMP THEN?

This is both extremely upsetting/annoying and false advertising. Please responsd/comment. Is there any way round this? Why did you do this (answer I guess to get people to buY the Sonos Sub...)

The AV installer I work with said he has set things up like an Amp and with third part sub-woofers before with the I guess first gen of the Amp with no issues. This just sucks. Feel like I wasted $699. 

AMP USER GUIDE

Enhance your TV sound

Connect Amp to your TV to create the ultimate entertainment system. 

• Use Amp and a pair of 3rd-party speakers to create two front audio channels. 

• Use Amp with third-party speakers to add surround sound if you already have a Sonos home theater speaker (Beam, Playbase, Playbar, or another Amp) connected to your TV.

(For even deeper sound, add a Sonos Sub or connect a third-party subwoofer.)

I have a related question… I just bought an Arc and second Sonos Sub to use in a Dolby Atmos surround setup with two One speakers in the rear. Sounds great as a pure Sonos system, but even with two Sonos Subs I’m really missing the impact of my 700W 15” Triad Silver.

I’d love to integrate the Triad (presumably in place of the two Sonos Subs) using the Sonos Amp’s sub output, but will that work for sub in an Arc+One Surround setup?

Even better since the Amp has speaker outputs would be the ability to connect an extra pair of speakers such as high in-wall for additional height effects, but I understand that is much more complicated than simply integrating the third-party subwoofer.

I like the Arc and don’t wish to give it up just to be able to add my Triad sub into the system.

Surround sound guidelines and limitations

Limitations
The following features will be disabled on Sonos products used as surround speakers:

AirPlay
Line-In
Subwoofer output
The microphones will be disabled on voice-enabled products used as surround speakers with a voice-enabled Sonos home theater product.

No.  You cannot add the Amp to the Arc+surrounds room, you can only group them together as two rooms.  And if you grouped the Amp with the Arc room, TV sources will be delayed, causing the Sub to lag, and the Amp won't be getting the .1 signal, it will be getting stereo.

I’ve been looking into a similar situation because I have an existing SVS 3000 sub and Sonos Arc.  I believe the HD Fury Diva can split the audio to both an HDMI eArc and RCA.  Not sure if the RCAs are fixed or line level. If they are line level and simultaneously output then this might be the solution for folks that have expensive existing subs.

edit: Page 107 of the HD Fury Diva manual says simultaneous output of analog L/R stereo

I assume you mean “fixed or variable?” In either case, how would you maintain the sub’s volume  relative to the Arc’s volume? 

Correct. Variable volume level that would be controlled by CEC.  The sub gain would be handled manually using the sub’s volume vs in app like on the Sonos Amp.  The open question I have to HDFury is if the analog out is variable.

I do have a spare Sonos Amp sitting around that I could feed another HDMI, but that is a costly solution just to use a 3rd party sub with the Arc.  HDFury Diva/Vertex2 + Sonos Amp would put the cost into the range of purchasing a proper AV receiver. I’d prefer to save my extra Sonos Amp for either outdoor speakers or for the gym.

I don’t think this is accurate.   Diva can extract ARC/eARC audio signals and send them over normal HDMI, optical, analog L/R stereo (RCA), but I don’t think it can actually passthrough an ARC/eARC signal from the TV to a Sonos Arc.  

The are other ways to do this though. You can use a receiver in between the source and display  (HDMI) that could power a 3rd party sub...assuming the receiver still sends an audio signal and that it’s a codec the ARc can play.  The other option is to use a Diva, Vertex2, or any HDMI matrix to split the signal to two normal HDMI...one to the TV and one to an Arcana that will feed the Sonos Arc.  Alternatively, you could get the new HD Fury Vroom, which effectively combines a Vertex2 and Arcana into a single device.   All of these have a the problem that you are playing the Sonos speakers and third party amp in a way that they will not be properly synced.   It might not be noticeable, but will definitely be the case since there is nothing in place to guarantee they play in sync.  I probably would only try it if I had use for one of these options outside of getting a 3rd sub in the mix.

You’re correct. The Diva / Vertex2 does not convert the audio signal. So I guess I would not get stereo if the source is Atmos. It looks like the Vroom does convert audio to the highest format for each output. Still not sure if L/R analog volume control is possible through CEC (i.e. the volume changes with a change in the CEC network).

I was even really thinking about the possiblity of the audio taking the lowest common denominator.  I have a vertex2, and I don’t think I’ve really seen that be the case.  Effectively, my two outputs are the Arc and a small TV that would be 2.0 PCM...although the sound is always off and I’ve never looked at what audio it’s getting.  The Arc plays atmos just fine.

As far as CEC commands, I think it if originated from a source remote control, like an Apple TV, then perhaps the HD Fury device will send the command to both audio systems.  However, if it’s the TV remote, I can’t see the command going to the audio system it isn’t directly tied to, if that makes sense.  I say this as Diva/Vertex2/Vroom are essentially 4x2 matrix that should allow you to essentially operate 2 separate with audio/video combinations (call it A1 / V1 and A2 / V2) with the same sources.  A CEC command from V1 should not reach A2.