If you connect the ARC to your TV it will output the front center, left/right and (Atmos) height channel audio and the two One SL’s will play the rear audio channels.. all are bonded together. You can also add up-to two Sonos Subs only to that ‘bonded’ setup.

If you were to ‘group’ those with an Amp/Speakers/3rd-Party ‘active’ Sub, then you would experience an audio delay due to the group buffering of the audio, which would (highly likely) cause an echo in the room and the Amp would just receive stereo audio only and be out of lip-sync with the video on screen.

The Arc/Amp setup in the way you describe it/are perhaps looking for …cannot be ‘bonded’ into one entity to play TV audio in surround sound sync, so grouping would be your only option. The ‘grouped’ rooms, whilst they won’t work for TV audio, will work fine for stereo music audio output, but I assume that’s not what you’re looking for in this instance.

Forget the Arc.

Just get the Amp, plus 2 x One SLs for the rears. Re-use your existing speakers for the fronts, plus your existing sub.

The Amp presents the centre channel from a surround mix as a ‘phantom centre’, using the two front speakers.

The one thing that’s not mentioned here is that the Arc is not a center speaker, it’s an integrated Right, Center, Left speaker, and those channels can not be split out to be sent to other speakers.

I’d follow ratty’s suggestion in your case. 

Thanks for these insights, I’ll start with the Amp and 2 SL’s and use my existing B&W for the fronts and my 25 year old REL strata 2 sub (which i simply cannot get rid of). Appreciate it.