How to stream with a pre-amp

As long as you don't crank up the AVR to Max, There shouldn't be a problem. Yes, the Saga will just "see" it as another source.
Yes, you would plug your CD player into the Saga and output the Saga to your amp.
For an all-in-one solution, the Bluesound is supposed to be quite nice.
What are you using for a streamer now?

Thanks - that helps.

In terms of service, mostly Spotify, running the 320kbps. The Marantz AVR houses everything - it streams. Here are a few notes about the Marantz: 

HEOS, Bluetooth, Wi-Fi and AirPlay 2 Capability

Dual antenna system lets you stream the most popular music services via Bluetooth, AirPlay or the HEOS app. Listen to your playlists on Spotify, Pandora, TuneIn, Deezer and more.

Here is a link with a good summary of its skillset. I'll note the the Saga was an immediate and significant step-up with respect to soundstage, even with the stock tube, which I understand is weak compared to other options. 

https://www.audioadvisor.com/prodinfo.asp?number=MASR5013&msclkid=079bdb1ae13a1974bf5d8672f6aa69...

Then just use the Marantz as your streamer.

It works, beautifully. I thought the added unit would cloud the signal, but not so. Thanks again.