What model is your amp? Does it have sockets for tape in/rec out? You can think of Port as operating in a similar fashion to a tape deck.

Does your existing stereo have tape in and out? If so, the Port’s out would go to the tape in, and the Port’s in would go to the tape out.

Note that with a Tape In/Out setup, if you play your Port grouped with other Sonos speakers (trust me, you will buy more), there will be a delay on the Sonos speakers compared to the stereo due to buffering.  If you really want to group them with no delay, you would need to connect the turntable to the line-in on the Port (with a phono pre-amp) and just rip all the CDs, because nobody really uses CDs after they get Sonos.  

My amplifier is one Arcam A90 with a tape in/out sockets, so I should be in good shape with the Sonos Port ?

NB: call me old fashion but I have tons of CDs that I love and listen to that are not on Spotify…   ;-)

So did I, so I ripped them to a disk and play them as a local library.  So much easier, and you get album art, track info, you can put them in playlists, etc.  

That Arcam has a tape monitor facility, which means that the amp could play the CD/vinyl/radio signal which is being “recorded” after it’s been looped out and back through the Port. This would ensure that the amp plays in sync with other grouped Sonos speakers.

Only issues I ran into with a similar setup was the AVR’s fancy internal processing adding a bit too much delay when using it with multiple Sonos in the same area. Turning it to “direct” mode fixed that.

@Stanley_4 where can I find the “direct” mode?

Stereo amplifiers do not introduce the kind of delay that AV receivers do and therefore do not need to be run via direct mode; and many do not even have such a mode. Are you seeing music play being out of sync when you wire kit per the above suggestions?

Another advantage of the ripping scheme/playlist recommendation is that it allows you to easily overcome the universally common tendency to listen to 20% or less of your CDs 80% or more of the time. Put all of one kind of music in a playlist and hit random shuffle mode and away you go.