Think of the CONNECT as just another tape deck. Audio out on the CONNECT goes to audio input on the amplifier. Audio input on the CONNECT goes to audio output on the amplifier. 

CONNECT’s Line-In is not automatically passed through to its Line-Out. Actually, Line-In is simply a music source on the network and Line-Out can play any source available on the network. Line-In must be selected as you would select any other source, such as Internet Radio play.

Thanks. I ran RCA cables as suggested. And  the Sonos app is now recognizing the Line In component as “Connected”.  So, progress! But still nothing coming thru (dumb) speakers. I’ve played with Line Out settings. Are there other settings I need to change? Or should troubleshoot with?

I can see the device on the Browse menu. 

It’s not just the speakers that are dumb here.

There is a FAQ here. It’s slightly sparse, but there are further embedded links. I tend to use either AirPlay 2 or Bluetooth on my system, all my vinyl has been long since replaced or ripped to my hard drive, and I use TuneIn or iHeart Radio plugins for any radio….all saying I no longer have an amp in my system .

Thanks v much for your help and patience 

Being a bit basic If you are trying to play a Sonos stream to the dumb speakers then you need the Connect output to be connected to an Input on the Marantz, that input needs to be selected on the Amp. The Connect must either initiate the stream or be part of a Group that is playing something. For testing purposes it might be worth setting the Connect to maximum volume and the Amp to minimum volume and then gentle turn the amp volume up until you hear sound.

Once it is working you might want to set the Connect to fixed volume (under settings) and control the volume from the Amp rather than having both volume controls active.

I gave it a whirl! Still nothing. I appreciate the basic walk thru. I double checked all connections. Will give Sonos customer support a call. Appreciate this communities efforts 

@Ralpfocus I continued to play around with Line Out level settings and I am now getting sound thru speakers!!!, albeit very quietly even when amp is turned all the way up.

so wanted to say thanks again for the advice. It did make a difference. 
 

I have a feeling I have a compatibility issue with Amp and Connect but, everything is at least connecting. I learned something today 

If the audio is just quiet when playing from the ‘Connects’ line-in, then try increasing the ‘source level’ in the Sonos App ‘Settings/System/�Connect Room Name]’ - and of course check the volume levels of both the Connect and the Marantz Amp.

To make things simple, forget about Line In settings for the Connect for now, and do not wire anything to the input jacks on the Connect.

If you wire the analog out jacks of the Connect to any line level input jacks on the Marantz via RCA cable, select that input on the Marantz via the input selector on it, and get music to start playing on the Connect via the Sonos app, you should hear it quite loudly on the speakers connected to the Marantz if the volume slider/knob on both Connect and Marantz at set to the halfway mark.

Move to using the Line in Jacks and the line in settings on the Sonos app for the Connect only after this first simple step is successful.