A DAC is a Digital to Analogue Converter. Just to understand your question. So you’d feed the analogue stream from the DAC “out” into the Port’s “in”. The “out” on the Port is connected to the power Amp’s “in”?

But now you want to differ volume behavior on the Port between DAC and Sonos sources? For Sonos sources you want the Port to control volume and for the “line in” source, the DAC, you do not want the Port to control volume? I do not think this is possible.  The Port either controls the volume on every source or it doesn’t.

You do know the Port also has a DAC? And that it digitizes any analogue stream that comes in before putting it out in analogue form again?

Sorry for the late reply.
What I would be happy to achieve is the following configuration.

PC-[usb]-USB Headphone DAC-[Line out/in(RCA)]-SONOS Port-[Line out/in(RCA)]-Power Amp-[Speaker cable]-Passive Speaker

1. When I want to play music from a PC, I want to adjust the volume using the volume on the USB Headphone DAC. This is because it is troublesome to adjust the volume with the SONOS application. In this case, SONOS Port does not have to do anything, so I would like it to simply pass through without changing the volume as well.
2. In addition to that, we would like to be able to casually use SONOS Port to play music from the SONOS app or spotify when the PC is not running. In this case, I want to adjust the volume from the SONOS app (since the PC is not running).

>You do know the Port also has a DAC? And that it digitizes any analogue stream that comes in before putting it out in analogue form again?

Yes, My question is whether it would be possible to take analog audio input from the RCA in and output it from the RCA out with pass-through and no volume adjustment.I am currently using a good quality USB DAC, so I would like to use that for D/A conversion if possible, but I'm not sure how difficult that would be.
But as you say, SONOS Port itself has a Degital in, so I may be able to get another connection configuration.