Hi. What do you want to achieve? If you just want turn your receiver into a Sonos player then you just need to connect one of the Port's outputs to a suitable input on the receiver.

What I’m trying to achieve — use of Sonos App to play internet radio stations on my legacy Niles multi room speaker system. Doing an upgrade to my legacy surround sound system w/existing turntable using two Sonos Amps. Is there an output on Amp I can use to get access to those internet radio stations for the Niles w/o purchasing a Sonos Port?

Sonos Port will do that for you.

As I just mentioned, can I achieve this with an output on a Sonos Amp (I’m getting two to upgrade my legacy surround/turntable setup) w/o needing to purchase a Sonos Port?

AMP does not provide a line level analog output.

You must have added the second question in edit.  Anyway, the answer is essentially no, as the Sonos Amp doesn’t have any RCA output.  There speaker wire to RCA conversion wires/boxes out there, but I’ve never used those before, and obviously, you would no longer be able to use the Amp to power speakers.  A Port is your best option, and this scenario is exactly why the Port exists.

FYI, when using a Sonos Amp for rear surround audio, the RCA inputs and sub output is turned off.  You’ll want to connect the turntable to the Amp you’ve got connected to TV, running front channel speakers.  You are using an Amp for the front speakers, correct?  You were not trying to say you want 2 amps for the rear surround audio, which is not supported by the way.

A speaker to line level converter would introduce operational complications -- especially if speakers are also connected to AMP. The most significant issue will be the “warring” Volume controls. (Niles and SONOS) Plus, you probably will need to add a switch to turn OFF the speakers. It would to possible to add yet another Volume control in the speaker wires -- now there would be three Volume controls in this “war”.