For the surround speakers, you could power them with a Sonos Amp, which would be ‘bonded’ to the Arc Ultra. There is no way to use a third party Subwoofer with a Sonos soundbar, I’m afraid. 

OK, pending arrival of my Arc Ultra, I connected the Amp to my in-ceiling speakers, grouped the Amp with one of my Era 100s and played some music from Sonos Radio. No sound from the in-ceiling speakers. 

The only wrinkle that comes to mind is that I’m using the banana plugs that were already attached to the speaker cables rather the banana plugs that came with the Amp. Can’t imagine why that would be a problem, though.

Thoughts?

Very, very unlikely that the banana plugs are the issue. Connect the ends of a flashlight cell to a speaker wire. At the instant of connection there will be a “thump”. If there is no thump, the speakers are not connected to the wire.

Yeah, it wasn’t the banana plugs. It was the fact that I had plugged in the patio speakers instead of the surround speakers -- because the speaker wires were mislabeled. Everything works as expected now. Hope I can say the same later today, after the Arc Ultra is installed and paired with the Amp.

Thanks for the quick response(s)!