Check, particularly, for 5 GHz interference between the Sub, and the speaker it is bonded. Although any electric interference is possible, as explained in the wifi interference FAQ. Of course, the ‘WiFi/radio’ must be turned on on both speakers.

If it continues to occur, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Thanks for the help on this - and I will generate and send the system diagnostic and take up directly with Sonos Support when they reopen.  Doing a little digging last night I now realize that the Sub needs to be relatively proximate to the AMP so they can communicate over their 5 Ghz radios.  I found a number of threads where users complained about this -- because many of us are replacing old amplifiers that drive speakers that have a home-run to a basement a/v cabinet (as is my situation).  I had thought the Sub communicated over the WiFi network or at least the dedicated Sonos net (I have both) - where physical proximity wouldn’t matter.  Interestingly, I have had no trouble with the Sub Minis that I have around my house - several of which are also connected to AMPs.  Hopefully the Sonos experts can help me fix this or the Sub will become a boat anchor and I’ll have to downgrade to a Sub Mini that at least will hold the connection.  Thanks again!

They will talk across Ethernet, but you would still need to have both devices wired to your router…and pay the (relatively) minor time cost for adding another routing device between the two Sonos devices. 

SonosNet is effectively just a hidden WiFi channel/signal, and as such, is subject to the same issues that affect any radio signal. And as the Sub connects across 5Ghz, as you have discovered, the distance it can reach is less than a 2.4 GHz connection, and more impacted by intervening materials. 

I look forward to reading what you hear back from Sonos Support.

Thanks Bruce!…. I wish I could connect them via Ethernet but we recently completed a renovation and I’ve got no way to cleanly pull cable between the 2 locations.  I have an old shielded RCA cable on that run that used to power my analog subwoofer, but haven’t found a creative way to Bogart the RCA cable to carry Cat-5 signals.  If I had such an adapter, my problems would be solved.   Will keep you posted on what the Jedi’s say.

Update from my call with Sonos Support
Confirmed that the issue is lack of signal strength between AMP and SUB due to placement (which cannot be improved since AMP is home-run in a rack in the basement).  Signal was “22 out of 100” per tech.  (Devices are 8 feet apart, but through flooring.)  Asked whether Gen 4 sub is any better (would upgrade from Gen 3) and it’s not clear there is any improvement (to warrant the cost).  Decided to swap Gen 3 SUB with SUB MINI (brand new, from other room) on theory that tech is better in newer products.  No better.  Now SUB MINI connects intermittently but won’t hold connection.  Not possible to hard wire with Cat-5, which is only other solution according to support.

I have concluded that the design / engineering limitations of these Sonos products ultimately limit or exclude their use from a very large use-case population, those of us with rack-mounted A/V systems in a room other than where the SUB will be located.  Very disappointing.

Consumers should be more aware of these limitations before purchasing Sonos products for this use case.  The impression is that all connections are made over either your home WiFi network or dedicated Sonos.Net -- whereas the reality is device-to-device connections are mandatory in some cases.  Hopefully the products will improve further over time -- in the meantime, buyer beware.

I’m not convinced it is a very large use case population, but I don’t have access to figures, either. And in a setup such as you describe, I suspect it would normally be relatively easy to run Ethernet cable back to the router.

But yes, people need to be aware of the limitations of WiFi, before purchasing network based devices.