Could you clarify your post on the following points:

“I have a preamp. Put in a Sonos Amp connected to two Sonos 5s for surrounds.”

How exactly are you using this set up? Is the preamp used for a turntable? Is the Amp cable connected to non-Sonos speakers? Do you use the Amp for your front speakers an Sonos Five’s for surround? Is the preamp connected to the Amp? How?
 

“I have a RCA out for a 2nd sub and want to connect the Sonos Sub to it.”

You have an RCA out on what? You know the Sub can only be connected wirelessly to another Sonos device (the Amp in your case)? Would (if you’d use another non Sonos sub) an RCA connection be capable of driving a sub?

”I had hoped the Amps sub connect was an in, it is not. Anyone have any ideas on how to connect a sub out to an Amp or Port to get it to the Sub”.

A “in” for a sub on an amplifier would, in my opinion, be a very strange concept. Why do you insist on connecting the Sub to an out instead of connecting the second Sonos Sub wirelessly to the Amp (with a Gen 3 Sub this is possible)? See https://support.sonos.com/nl-nl/article/set-up-your-sonos-sub

Do you know know the Port cannot be used to drive a Sub?

I’m sorry but I cannot work out what your setup is or what you are trying to do.  Perhaps you could elaborate / clarify?  In particular:

What sort of preamp?  What is its role in your setup?

In what sense “connected to”?  The Sonos Amp requires passive speakers so you cannot use Fives with it.  Or are you saying that yu have an Amp + passive speakers for the front, with the Fives wireless as rear surrounds

The Sonos Sub can only connect wirelessly

Sorry, I can’t get this at all

To provide clarity on the configuration @John B and @106rallye 

I have a NAD M17. It is connected to tow tower speakers (fronts), and I have a Center and a sub on order. All of these are hard wired in to an amp using XLR. 
 

The M17 is a preamp, and supports up to 11 channels. 
 

So I ran the surround via RCA to a Sonos Amp, which transports the signal to two Sonos 5s that are acting like surround L and R. I am going to swap out the Sonos Amp for a Sonos Port as I do not need the hardware-to-speaker aspect of the Sonos Amo. 
 

The M17 supports two subs. I would like to figure out how to do what I have done with surrounds (Sonos 5s via the Amp/Port). I have a Sub2 out (Sub RCA). I thought the Amp would let me do this - but the sub connection is a sub out, not a sub in. 
 

Maybe it is not possible. 

Apologies for SPs ….typing on phone!

If the Amp’s sub connector is an output, do you not just run a wire from it to your sub? Or use the second sub output from your M17 to your second sub?

@nik9669a  the Sonos Sub does not have a physical connection. I am trying can I take a sub connection (out) from the amp and get it to the Sonos Sub.

Sorry- thought it was a TP sub. Do you not simply “Connect Sub” using the app (under Settings/System/roomname) and bond the Sub wirelessly to the Amp? No physical connection needed. 

Any wired sub output is intended for a non-Sonos subwoofer.  It cannot be used with a Sonos Sub.

And I am afraid I am still totally baffled. 

This will not work well. Any Sonos device using an analogue in will add a 75ms delay. So using Amp or Port to get other Sonos speaker as surround will get you a delay to the surrounds. That will sound awkward.