Those aux inputs seem pretty tempting to connect to the analog outs on the Sonos. The antenna is obscuring some of the tape recorder data, it might be helpful if you could provide a link to an online manual for this device.

Ah, I think I’ve figured out the tape loop, it’s oddly separated, which confused me a bit. I would connect the analog out on the Port (or CONNECT, you haven’t specified which one you have) to the Record 1 RCA jacks, and the analog input on the Port/CONNECT to the PLAY 1 ports. 

Plugged RCA from play5 into both left inputs under RECORD and somehow that worked… odd

You must have a first gen PLAY:5, the second gen doesn’t have a headphone output, only the line in. 

If you are attempting to play output from the SONOS system through the Marantz. Connect the output from PLAY:5 to any input on the Marantz except PHONO. I realize that one of the Marantz RECORD outputs might work, but this is an accident and there will be side effects.