It depends on how good the amplifier is.  Do you already have a pair of passive speakers?

The Sonos Amp sounds the same as any other amp from a known brand, that has approximately the same power delivery spec. Getting a port only makes sense if you already own such a third party amp.

PS: The amp you may own need not even be as powerful as the Sonos amp, for it to serve well enough. This depends on how much power your passive speakers need to perform to spec.