You cannot bond the Sub to just the Port. The only way to use the Sub with the Port is if the Sub is bonded to your Arc and you have your Arc/Sub grouped with the Port.

Hi @BP86,  

Welcome to the Sonos community. Thank you for reaching out to us and for letting us know about your concern. Let me share some information with you.

The Sonos Sub (Gen 3) can be bonded with any Sonos speaker, with the exception of the Move, Port, and Connect like @GuitarSuperstar mentioned in this thread. Both Connect and Port are not compatible with Sub (Gen 3) as a way to prevent volume conflicts and ensure the best possible listening experience. The Sonos Move is a portable speaker that you can bring around your home, either outdoors or outside of the home - and away from where your home theater set-up is, which is why it is unable to bond with any of our Subs. 

Hope this helps. Please feel free to reach out if you have further questions or concerns, we’ll be glad to assist you. 

I think you need a Sonos Amp, not a Port, and everything will work perfectly.  But your existing amplifier would be redundant.