I will assume that the Sub is not cabled (it’s best if it isn’t). 
My thoughts are it’s probably worth checking the settings on the managed switch for the Spanning Tree Protocol. Perhaps see if this support link assists:

https://support.sonos.com/s/article/2118

Also make sure that you have not switched off the WiFi adapter on the Sonos Arc, as the HT/Sub communicate with each other directly over a 5Ghz ad-hoc wireless connection.

Thank you for the fast reply. I’m not an IT person, but I’ll dig around the managed switch settings to see what I can determine. I suspect it’s that the ARC isn’t happy about the type of wired connection it has. With that theory in mind, I changed the setting on the ARC to disable the Ethernet connection and go with Wi-Fi…and presto, my sub woofer came alive! I’d rather keep the ARC wired, so I’ll have to dig around some in the managed switch settings to see if I can get it working right by having the ARC wired while still able to communicate with the sub.

Still, when it was working, and I put my ear up to the rectangular hole, I could still hear a very faint fluttering sound- what’s that all about??? I expected it to be absolutely silent except for bass.

Not sure what the Sub fluttering is, unless it’s coming from the power supply - maybe try a completely different power socket to see if that gets rid of it.

Just make sure when you wire the Arc that you do not switch off its WiFi adapter as it’s still required for Sub/Surround connection.

If you are unable to resolve the managed switch settings after reading that Support Page link, then maybe post the make/model, or better still, a screenshot of the switch settings, as someone here in the community (hopefully) may know the best settings to put in place or of course keep life simple and opt to use an ‘unmanaged’ switch instead, if you have one available.

Oh, hang on here…maybe everything is fine then. I DID have my Wi-Fi disabled on my ARC as I thought that’s what one was supposed to do if you wired it. Are you saying that it’ll make the wired connection if I plug it in AND it then communicated via it’s WiFi to the sub??? If so, then bonehead me, I assumed that disabling the WiFi on the ARC was the way you got it to use the LAN connection…doh!

Yup: Arc connects over a dedicated 5Ghz wifi link to Sub and/or any surround speakers, so wifi must be enabled. 

It perhaps might help if there was some information displayed in the Sonos App, that informs a user that when a products WiFi adapter is disabled, that it may cause that device to have some communication difficulties with other Sonos devices when it forms part of a Home Theatre setup, or words to that effect …and if it is part of a HT, to consider/recommend it might be better to leave its adapter enabled.

The guidance around this issue has been slightly problematic for quite some time.

'Slightly problematic' is a generous understatement.  Why Sonos have not changed the labelling of this setting remains a mystery to me.

This was user error, but one that is entirely understandable.

I’m wondering whether the perfect might be the enemy of the good here. Imagine if someone in product development has been charged with making it impossible to disable the radio under such circumstances (just as an attempt to disable on a device without a fully wired router path fails). If so, they could be tying themselves in knots trying to cater for every scenario. A shame if that’s the case, when a simple re-wording or explanatory note in the controller would suffice...