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.