Hi @keg987.

Welcome to the Sonos community and thank you for reaching out to us.

I would not recommend hiding the Sub in that situation as it may alter the Sub frequency sound that the sub is generating. The cavity that encloses the Sub in that situation may absorb the sound wave that the Sub is generating thus will make low to no effect on the Sub functionality. We have to consider that the Sub, Majority of the time is felt via that Sub frequency wave than heard. I would suggest corners or right below the TV if you have a table or fireplace, right at the side of the table or fireplace would do (not close enough for the heat to affect the Sub). Beside the couch would also work. these are the recommended setup of Sonos for the Sub.

I hope this helps. 

Please let us know if you still have further questions or concerns. We are always here to help.

Thanks,

The sub is gorgeous! Why would you want to hide it? I really recommend getting it. It really makes a huge difference. It is very smooth and nice to listen to. Adds a lot to the movie viewing experience. 
 

(Some people lay the sub on its side, and slip it under the sofa. Personally, I prefer to display it prominently as it’s really beautiful and can match any room decor.)

If you’re buying a Sonos Amp, you can wire its subwoofer output to *any* powered sub -- you don’t need to use the wireless Sonos sub.  There are some really impressive (and, alas, expensive) in-wall subs, like the JL & James units.  There are some inexpensive ones too, like Monoprice and OSD, but I don’t know if they’re any good.

Thanks for your replies @Paul A and @tracker . I’m going to look into the in wall powered subs. Do you know if I can connect a 3rd party powered sub to the Sonos Arc directly or if I need an amp? I’m planning on using the amp to control 2 in-ceiling speakers as surrounds and I saw somewhere I can't also use it for the sub if I set it up this way. It’s simply not an option to leave it visible or under the couch so its either this or maybe in a cabinet then.

Unfortunately, no. You can’t connect a 3rd party sub to the Arc and as you correctly state, you can’t connect a 3rd party sub to the Amp when using it to drive your surrounds. 
 

Have you considered going for a third party receiver from Onkyo or Pioneer and then going all in on build in speakers?

One receiver will easily drive a full Atmos setup with build-in speakers and because it supports Works With Sonos, you connect a Sonos Connect or Sonos Port to it which will enable you to fully control the receiver as a Sonos speaker - including turning it on and on, switching music, grouping and (with the latest update) control volume via the Sonos app. 
 

I am doing that in a new house I’m building to avoid any visible speakers (all my speakers will be in-ceiling with pivoting tweeters and mids to compensate for placement). 

Thanks @Tejs Dyrvig Ernst I hadn’t considered this because the Sonos Arc looked so good and easy to use that I had planned to go that route instead of the 3rd party receiver route. tbh I also didn’t know about the works with sonos designation. I just hadn’t anticipated how difficult it would be to place the sub in the room in a way that makes everyone in the family happy. 

There is a wall mount available for the sub, so it’s location can be a bit more varied, especially if you’re trying to get it off the floor. Will you have any shelves that it could go under to make it less prominent? Or make a feature of it - it is quite elegant!

Give it some thought. I am sure the family would appreciate all of the speakers being out of sight. With a receiver, you are also avoiding much of the frustration people are having getting Atmos to the Arc, as sources will just plug straight into the receiver, and of course format support is wider on receivers and includes DTS as a notable omission on the Arc. You also get HDMI-inputs on the receiver, so only on cable is needed to connect all your sources to the tv. 
 

It wouldn’t necessarily be more expensive either. One receiver can drive up to 9 speakers and a sub, so instead of

1xArc, 1xAmp, 1xSonos Sub and 2x speakers

you would need

1xReceiver, 1xConnect/Port, 1x third-part sub and 4-9x speakers at your choice.

The price of the Sonos Amp is more or less the same as the receiver and a third-party sub is likely to be cheaper or at least not more expensive than the Sonos Sub. Take a look at the Onkyo TX-NR696 or Pioneer VSX-LX304 receivers for instance. A used 2nd gen Sonos Connect is likely in the range of 100-200USD. 

Are there any workarounds to an in-wall sub home theater system?

Currently, I have a Sonos Arc and two rear in-ceiling Sonance speakers connected to the Arc through a Sonos Amp.

I also have a wired in-wall Sonance subwoofer with a separate episode amp to power that subwoofer (I think both were pretty expensive).  I believe the installer thought that the sub port on the Sonos Amp would send a signal that could be amplified to power the Sonos sub.  It doesn’t seem to be the case.  Is there any work-around that anyone is aware of to get this subwoofer to work?  If I purchase a separate receiver and sonos port to power the rear,  that wouldn’t help, would it?