If you have an iDevice, you can Trueplay the system to fine tune it.  If Android only, there is a sub output adjustment under Settings > System > (Room Name).

The same thing has happened to me, except I just replaced the Beam with the Arc. Although the system says it is working, I am getting no audio through the sub. This is very noticeable because I have enjoyed it for over 5 years, so frustrating 

Check to make sure WiFi isn’t turned off on the Arc. It needs to be on, even if the Arc is wired to Ethernet. 

It's turned on. I managed to get it working again by connecting the sub to the router with an ethernet cable, although it's only been on for a few songs. Tonight is the second time it's happened since I got the Arc last week. First time I only had to remove then reconnect the sub but that didn't work tonight despite several attempts. I'm used to having the sub next to the TV but if it needs to sit behind me connected to the router I can live with that.