I’m a little confused on how you worded your setup.   The Port is essentially an audio source to be used with amp.  The port is not not ‘driven’ by an amp the way a passive speaker is driven/powered by an amp.  You would not need to use multiple ports for a multichannel amp, unless you want to have different audio in different zones that the multichannel amp is serving.

As far as volume control, you can configure the Port to output a standard volume level for audio inputs, thus the amp would completely control the volume.  Or you can can set it with adjustable volume, so that you can control the volume through Sonos and the amp.  

Thx Danny.   my apologies for the confusion in my question.  

I’m looking to use, as example, a 6 channel separate amplifier I already own, to drive 3 ports for 3 separate rooms.   Will I be able to control the volume in each room separately via the sonos app, and with no decrease in capacity of amp output into those rooms.  

Each Port can be independently controlled. 

And, again, the amp doesn’t ‘drive’ the Ports. The Ports drive the amp. How you direct the output of each Port is down to the amp configuration.

power.   not drive.  :)   

Hopefully to clear things up.

The Port will receive power from an electrical outlet.  Your amp receives power from an electrical outlet.  Your passive speakers receive power from the amp.

The Port will be an audio source for your 6 channel amplifier.  If your amplifier can use multiple audio sources, then you can connect multiple Ports to it.  How the amplifier uses those audio sources is up to your amplifier.  Most I’ve seen will allow you to send different audio sources to different zones/rooms, but don’t know your specific amplifier.

Volume control is as I stated earlier.  You can have it set so the Port(s) use a standard flat volume, no control on the Sonos end, or you can enable variable volume control through the Sonos app.  Your choice, whatever works best for your scenario.