See below.. 

There is RCA Analog audio in/out and Coax digital audio out only. There’s no digital input on the Sonos Port.

Assuming the below image is correct, it seems the way forward is to use the RCA in/out analog connections, depending on what you’re looking to achieve…

Thanks for your reply Ken. Trying to stream my old cd collection already ripped to the X12. Tried connecting as a form of NAS drive without success so thinking of going for the Port now and a pair of analogue cables  - out from the X12 and into the port. That sound ok before I outlay more cash into the Sonos coffers? Thanks again for your reply! 

Your community profile lists a number of Sonos products with a line-in, not least a Sonos Play:5 (gen2) and an Era 100 (which requires the Sonos line-in adapter) - can’t you just use either of those devices connected from your streamer RCA line-out to the line-in on either of those devices and not bother with purchasing the Sonos Port?… That’s unless you also want to use a Sonos audio-out link for some particular reason too, into your X12?

You just need an RCA to 3.5mm audio cable in the case of the Play:5 to achieve what you say you’re looking for.

Note I’m assuming the two existing speakers mentioned that you have, are not being used as HT Surrounds (which disables their line-in).

Edit:

RCA to 3.5mm audio cable (example): 

https://www.amazon.co.uk/Amazon-Basics-Adapter-Subwoofer-Gold-Plated/dp/B01D5H8JW0

Hi Ken. No Era 100’s are stand alone set as a stereo pair. My Five internet card sadly died a month or so back so is u/s.  You’ve given me food for thought though so will try your Amazon adapter cable suggestion first. Thanks again. Appreciate your input!! 

To add to the above - will also need the the line in adaptor for the Era 100 I realise on re-reading your post! Cheers again. 

I'd really look at the NAS option, it will likely be less aggravating long term.

The X12 is supposed to ‘talk’ to Sonos as they are both on the same network though the X12 manual is very vague on the matter. Hard wiring to the router is a non starter as all my ports on the router are already in use. Thanks for your input Stanley_4 as I’m willing to listen to any suggestion! As an aside - which has been raised before - why is the Port so expensive ??? !! 

Extra ports for a router can be provided for by installing a switch, for Sonos preferably an unmanaged switch. I own a couple of these: https://www.tp-link.com/nl/business-networking/unmanaged-switch/tl-sg105/

Thanks 106rallye. More food for thought!