Presumably you connected the Sub to the Arc using the “Add Sub” function on the Arc settings? If you’re able to play it with the Play:3, it sounds like you might be trying to group the Sub with the Arc on the grouping screen rather than adding the Sub through the correct route? 

Yes, it pairs, but the sub doesn't make a pairing chime like it does when it pairs with a play 3. 

Just doesn't play anything 

I’m not sure I understand how your sub pairs with the Play:3? Can you explain how you are pairing it so I/we can maybe advise better on the pairing with the Arc?

In the same way, just go into the product on my system and select "connect sub"

Okay. Add Sub to the Arc, then increase the sub level and increase the bass in the EQ, play maybe a dance track, then put your hand in the cones of the Sub to feel them moving. This is the best test that all is well. The sub doesn’t ‘play’ anything, it just takes the low frequencies away from the Arc (and enhances them) to allow the Arc to better concentrate on the mids and highs.

It doesn't even play the connection chime when it pairs with the ARC, there's nothing coming out of it when connected to the ARC but a noticeable difference when connected to the play

I would suggest Removing Sub (and if that option isn’t there then something is wrong).

Then turn off the Arc and the Sub at the wall. Turn the Arc back on and let it fully boot up until it shows the white light. Then turn the Sub back on, and go through the Add Sub process again. It might be an Arc thing, that a restart will correct.

I had tried that, just tried factory resetting and reading the sub but now I'm getting the "can't check for updates" error

Factory resetting is worst case scenario and can scupper things further…

Are the Arc and Sub now back on the system at least…? If so, you can download the Windows or Mac app and run the system update from there which often helps. 

But if you got that error during the re-adding process, not sure what to advise. Often an error can be repeatedly shown, but then after trying multiple times, it ends up connecting fine...

Ok I got it readded, and I've got I've moved one of my mesh network nodes. So I had tried it on WiFi, which didn't work, and sonosnet with a play:3 wired.

Now it's working by moving a node and wiring the arc into ethernet.

Phew.