1. This is doable, but probably backwards from the way you are thinking.  All Sonos inputs have a minimum delay of around 75 ms. In order to make this work, you’d need to send the signal in to the Sonos ecosystem first, and then send the signal from the Sonos system to your amplifier. Assuming the amplifier doesn’t add any delay of its own, then the Sonos and the amplifier’s speakers would be in sync.

2. As in this case, the Sonos system is streaming to your amplifier, yes. 

Essentially, you want to connect the Port as if it is a tape player.

I have a similar question: 

Thinking of a way to connect a Sonos multi room set up to my Naim Atom. As the Atom has an RCA out could I just connect this to the Sonos Play 5, which has an RCA in? From there I could connect by WiFi my Sonos 5 to other Sonos on the system? Would a delay caused by the Play 5 mean this is not really a solution? 

Any analog on a Sonos device is exactly the same, except for the form factor. 

Since I’ve no idea what a Naim Atom is, I don’t know if the delay on a Sonos analog line in would be an issue or not. If this Atom has external speakers, then yes, unless you set it up as I indicated in my original answer, where it is playing the output from the Sonos. 

Thanks for weighing in.  I’m not sure I’m completely following, so I drew this little diagram which is what I had in mind.  Is this the best way to accomplish what I have in mind (‘any’ source playing through ‘any’ speaker)?  Maybe this isn’t technically how it all works but as a logical construct does it make sense?

So yes, as long as you never use anything on the amp in your diagram as a music source, and it only drives the non Sonos speakers, that setup would work. 

Most people try to connect the turntable and CD player to the Amp, which is what I’m cautioning about.

A warning, though. While the turntable will need the pre-amp to alter the signal from phono level to line level, the CD player does not. Here is a good reference between Phono and Line levels. All Sonos line-ins are looking for line level data. So, you’re going to need some sort of switch before the turntable/pre-amp and the CD get to the port, so you can choose which one you want to use. 

Naim Atom is an integrated amp and streamer. If I send this via RCA to the Sonos 5 it appears there will be a delay, so that ends that idea… 

Okay thanks, Airgetlam. My goal was to confirm that the Port will work for me, and I think you’ve done that.
 

This would be a lot easier if I just ditched the old stuff and got a couple of Fives, but I inherited a vintage 60’s McIntosh C22 and 240 from my grandfather.  It’s my understanding that this is the kind of equipment you don’t ‘ditch.’ And I’m emotionally invested in using them properly as a way to remember him and my grandmother.  

Thanks!

You would connect the Port to the tape loop of the preamp and the Port would process the signal from the turntable or the CD player for when you want to group that sound with your other Sonos devices.