Basic setup

The simple answer is a Sonos Port.   All Sonos products have access to your local library, but only the Port can send that audio to your integrated amp.  However, you could replace your amp with a Sonos amp as an option.  If you are willing up to give up your amp and passive speakers, you could replace with any of Sonos powered speakers, except for the Ace headphones.

Thanks for the quick reply and information to digest, however, I’m not ready to give up my MA6900 and my CM9 speakers quite yet.

EdP

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With a Port, you don’t need to, the Port has analog inputs that you can use for connection to your amp. If you’re familiar with the old Sonos CONNECT, it is merely the newer version of that.

I’d retire that BRIDGE ASAP, though. They’re renown for failing power supplies, and the results tend to be hard to diagnose. Fortunately, you can wire Ethernet cables directly to a speaker, and it acts in the same manner as a BRIDGE. Even the BRIDGE’s replacement, the BOOST, has now been retired. 

If I understand you correctly, the Port acts just like my Player 90, except I have a digital out from the Player to a DAC, then analog out to the amp. But how do I get the wireless signal from my PC to the Port?

PC to Wi-Fi router to Port.

Optionally and not currently recommended is to wire one or more primary Sonos devices to the main router and use Sonosnet instead of Wi-Fi. 

Okay ... now here's where I exhibit my ignorance of things wireless.
I have a printer across the room that receives wireless output from my PC/router. Why can't my Zone Player in the next room receive wireless signals from my PC/router without requiring an intermediate device such as a Bridge or Boost?

Not sure where you heard that, don't think it has been the case for many years.

All Sonos have a Wi-Fi radio that talks to the router.

Both Bridge and Boost are discontinued by Sonos