Improbable.  The Port relies on downstream amplification, which may or may not be controlled via the Sonos app.  So the Sub has no way of knowing the output volume of the downstream amp, and thus has no way of balancing the sound. 

I agree with @jgatie , except that I would rate the chances as zero, not improbable.

If the amplifier through which the Port is streaming has a subwoofer output then you could buy a non-Sonos subwoofer to complement the non-Sonos pair of HiFi speakers you are presumably already using.

Correct, but the Amp is the Marantz MM7025 and is truly power only and no Sub output or even pass through of the inputs that I could use to feed into a sub with own crossover.  Don’t really want to split the signal from the Port but it seems the only way.  I wouldn’t think it would be that hard to add a sub option to the Port as long as it was set to Variable output.  

… apart from the reason already given by @jgatie.

I’m sorry but I don’t know what you mean by this.  I know what a power amp is but don’t see the relevance.  I realise it closes off the non-Sonos subwoofer option, but that probably goes with the territory of an AV amp?

I am not sure what you are planning to do here, if you are still talking about a Sonos Sub

That Marantz is a power amp not an integrated amp or AVR so has no processing or preamp control like an AVR or integrated amp. It’s unbalanced rca or balanced in to speaker out.

With a typical AVR or Stereo amp, which has a built in pre-amp/processor then that can provide a subwoofer rca output when the port is connected as an input and has volume controls so the port can be set to fixed output.

For your specific use case, which is the same way I use to run a Port, direct into a power amp, it could be workable, but I suspect a niche use case within a niche product in the Sonos line up.

I would bet the majority of Port’s sold are plugged into either powered speakers, integrated amps or AVRs at which point you have the issue of which device is in charge of volume and signal processing. Some people will have the Port on variable volume for convenience, some will have fixed and use the external device for volume.

Rather than complicating the setup to try and cater for all the different combinations, the Port keeps it simple and in line with many of the traditional HiFi equipment… 2 channel for audio. It is only in recent years with some manufacturers that they have added hdmi inputs and/or sub outputs to their Stereo products. Even more shocking imo, is how many still ignore room correction and 2.1 for their stereo products.