I don’t think this setup is possible. Like all other Sonos sound bars, the Arc can only be used with the Sonos Sub. 

With an optical splitter, I’m using the Sonos Arc and self-powered sub.  I’m thinking about some adapters that might do it. 

I suppose it depends on whether the TV stops the optical output when using eARC. Maybe check with the TV’s manufacturer. 

Interesting.  How is the crossover handled?

I have two adapters. Optical splitter and optical to regular speaker wire. What I’m going to try is an HDMI splitter with a arc adapter to optical. 

No crossover, other than the low pass filter in the subwoofer. And how is volume to be controlled, unless it’s in the TV itself? 

Sub is self-powered with volume

So you have to adjust the sub volume separately every time you change the Arc volume?

The sub has two controls, one for volume and one for tone

What has that to do with it? I’m talking about how to keep the sub volume and Arc volume in step.

So you are going to get up out of your seat and adjust the volume on the Sub every time you adjust it on the Arc?  

No need, set and forget it.

You never change the volume on the Arc?

That won’t work.  The volume of the Arc is applied inside the Arc itself, so the signal split to the sub will have no way to know what volume to play.  If you “set it and forget it” it will be too loud for low Arc volumes, and too low for loud Arc volumes.  

Point of this post is to search an answer in splitting & adapting eARC and optical.  Wanted to make that clear before getting it derailed. Also, you can get some exercise walking to the sub if need be right?

How is it getting derailed?  People are pointing out how your proposed answer may give disappointing results.  Would you prefer encouragement for a plan that ultimately fails?  

Lmao. What I would prefer is you share any knowledge you may have (if any) on the topic of the post. 

Uhhh, that’s what we are doing.  Our knowledge is that your proposed plan won’t work for the reasons given.  

We’ll see about that ;)

Note the username. I’m out. 

See yah, ratty. I will share the fix when I find it here.

There are some interesting hacks around the internet using the Ikea Symfonisk to turn any powered sub into one that can be added to a Sonos zone. No crossover control mind you but looks like fun!

Would allow you to to adjust the relative volume.

There we go. That's what I'm talking about. 

You are welcome.

I have a similar problem, but my sub as well as existing ceiling rear speakers are powered by an AVR. I want to have my TV’s audio play out of the Sonos Arc and the AVR at the same time. I found this splitter that might do the trick, but I haven’t purchased and tried it yet: https://thenaudio.com/product/zone-2-earc-audio-processor/