Hi Glenn.  I am struggling to understand what you are trying to do.  Can you please explain what you mean by “using a stereo line level sent to the Ones rather than the app”.  The Sonos app is just a remote control for the Sonos system.  

You can use Subs with Ones.

I’m puzzled.

If you are looking for a Line-Level (like RCA plugs) input the Amp would work but it is really intended to drive passive speakers.

A Port is designed to be a Line-Level input but it is kind of expensive compared to an Era-100 and the Line-In dongle. You also end up with another speaker which is never a bad thing.

Keep in mind the Line-In has a 75 ms delay, no problem for streamed or local library music but it often confuses any live performer or speaker.

What do you want to use as a line-in source?

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.  

And you can't use passive Subs with the Amp. You can use Sonos Subs or third.pary active Subs.

Not sure whether the sync issue would matter much in practice in your case. It would depend on how attentively people were watching and listening, and on the nature of the content.

Thanks a million for all the info folks; greatly appreciated. I have enough to totally confuse the restaurant owner now! (JK!!)
I think I am going to recommend that we use conventional speakers and maybe he can put the Ones in his office!

Glenn

Grab a Sonos Port to feed music to the amplifier and speakers you select.