Not really. There’s no effective way to run a third party sub with a Sonos soundbar. While you can ‘group’ the Sonos Amp with the Sonos home theater room, which would keep the subwoofer output live, there would be a minimum of a 75ms delay between the home theater room and the room that the sub is in. 
 

Essentially, the only way to use a third party sub in a Sonos home theater context is if you’re driving your own front speakers (and no center speaker, just the pair) using the Sonos Amp. The Sonos system would interpolate the center channel, and the Amp’s subwoofer output woul be available. 

Hi.  I wasn’t aware that Sonos was a ‘soundbar company’, but if you are looking for consensus I can confirm that Bruce’s comments are completely in line with my understanding.

Where your thinking goes awry is in “If I buy the amp, I can pair it with both the arc and one sl's.”.  ‘Pair’ has no meaning in this context for Sonos. The surround speakers have to be “bonded” to the Arc, and only one set of surrounds can be bonded.  If you bond the Amp as surrounds (even with only your subwoofer attached) then you cannot bond the One SLs. And when bonded as surrounds, the Amp does not have the subwoofer connection active.  

The main takeaway is this: you cannot use a non-Sonos subwoofer while using an Arc for the front left, centre and right, which is all the Arc can be used for.

You could, I suppose, group the Amp with the Arc, but this would not allow crossovers to be handled properly, the balancing of volumes would be manual and tricky, and IMO you would be forever fiddling with it in order to achieve an unsatisfactory outcome.  And it would not sync perfectly for TV sound. It would work in the sense that sound would be produced.  It might even sound OK.  But if the idea of using a Sonos Sub would be anathema to you, I can’t imagine you would be happy with such an arrangement.

I’d be interested to hear why/how your subs are superior to Sonos subs (you can use 2 of them in an home theatre).  I’m not suggesting there aren’t better subs (or sub setups) out there than Sonos makes, just that if the quality of the Sonos sub is an issue for you, I’m surprised that you are ok with using a soundbar for your front audio channels.

Thanks for the responses. Looks like we will have to go a different direction. The Sonos sub I'm sure is a wonderful piece of kit. But, when you've been listening to dual Krell Reference subs, your mind simply will not allow you to try and adjust to less filling sound. If only Sennheiser had made the Ambeo with the option of running your own rear channels, we'd have been set. Oh well, the search continues. Thanks again everyone. Happy listening.

I think you live in a region where the Sonos “home trial” is active. If you don’t like the sound, return the kit for a full refund. So you could home-test the Arc and Sub to let your ears try out the differences, and return the kit if you don’t like it.