Think of the Port as another tape deck. Connect it to the Denon as a second tape player. 

got it - thanks - that is working..one more question, the volume on the Sonos speakers is really low - is there a way to correct that - I have tried variable, fixed and pass-through and they seem the same - any ideas? Thank you!

Sonos line in is expecting a line level input, essentially what all devices put out on their standard connections, save turntables, which are generally phono level, and require a pre-amp to get to line level. This is why most amplifiers have a special place to connect a turntable, the amplifiers tend to have a built in pre-amp for that purpose. 

You should never be connecting a variable input (headphones, speaker cables, etc) to a Sonos line in, you might conceivably blow out the input. At the best, you’d spend a lot of back and forth figuring out volume levels on each device to get the best quality. 

Not sure what you have the Port connected to, but I’d think ‘fixed’ or ‘pass through’ would be correct, although those are not generally options available on a standard line level output. 

thank you!