Hi; thanks for the replies. A little clarification:
The "line level stereo source" is the audio output of an HDMI matrix switch, so whatever is going to the tvs goes to the Sonos ones.
Unless I'm missing something it seems like the AMP is the way to go; and I can just put a couple of passive subs on it in addition to providing a stereo source to the ones?
This won’t work as you anticipate. Sonos buffers audio it receives from aux source, except for TV audio through HDMI-ARc ports. Even then, any room that’s grouped with the Sonos amp or soundbar that is connect to a TV audio source will also be buffered and delayed.
In other words, if you connect a line in source from your HDMI switch to the Sonos amp. the audio will not be in sync with the TV video. You could instead connect your TV to the Sonos Amp (this is the normal setup) and the Amp would play in sync with the TV, however grouping your Ones with the amp would not be in sync. The ones can be setup as surround sounds for the Amp, but they would play surround audio, not stereo channels.