Unfortunately I am not an expert.
But my Raspberry Pi4 is expanded with a DAC.
Would a WiiM Pro Plus replace the DAC?
This image was confusing me as it shows also a DAC.
Not enabling squeezeplayers will cause the RHEOS application to quit as no devices can be discovered found. It doesnât âcrashâ. Using on a NAS requires some reasonable familiarity with Docker containers and is not officially supported by me - but other members are kindly helping out here (thanks Sven 
The simplest installation would be to install roon extension manager on the raspberry pi and then enable RHEOS. Follow the instruction on the readme here once installed
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Regarding the WIM - I donât believe this will control the AVR any differently than your raspberry p with the installed dac.
rooExtend is for Raspberry Pi only. Please visit http://diy.rooExtend.com or check if the rooExtend-Box https://rooExtend.com might be an option for you.
Best DrCWO
Adding to Svenâs comment on grouping - you will only be able to group within the type of players (Sonos-Sonos, Airplay-Airplay, RAAT - RAAT and indeed RHEOS -RHEOS). The different protocols and approaches mean you cant synchronize reliably. Even if Denon/Marantz HEOS2 enabled devices become certified they will likely only group with other RAAT devices (we will see if this is the approach).
All you need is a DAC that supports USB Audio Class 2 (which most modern DACs) do and a Raspberry PI with either Ropieee or rooExtend. Both have a Roon streamer included so connect Your DAC to the Pi and the Pi to Ethernet and you will be fine.
Get this excellent and inexpensive DAC for example and get Sound that is state of the art 
There are also other excellent DACs measured on audiosciencereview. I personally use a RME ADI-2/4 but this is 10 times more expensive 
Best DrCWO
Thanks for trying to explain me. As I said: I am not an expert and I still do not understand.
Actually my Raspberry Pi is connected to my Denon AVR. Is then the AVR acting as DAC?
The WiiM Pro Plus is not bad as it already includes the Streamer plus anaog in if you need that.
On the other side a Raspberry Pi plus the Topping D50 III delivers more fidelity and you also can use the Raspberry for Roon Extensions.
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And is the Rasperry Pi connected to teh DAC?
Exactly 
The Pi includes the Roon streamer plus Roon Extensions and is connected to the DAC via USB.
I forgot to mention that the he Topping D50 III also has symmetrical outputs increasing the quality even more id your AMP offer XLR input.
In my eyes a killer combo for Roon Streaming with a killer price tag.
Can roopie and Roon Extensions run on the same Pi?
As far as I know roopie offers to run the Extenension Manager.
rooExtend has the Roon Streamer the Extension Manager and all my Roon Extensions on board. Running the Streamer and the Extension Manager on rooExtend is for free. If you additionally like to use one of my Roon Extensions you have to get a License Key. I recommend getting a Pi4
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Which software did you run on the Pi?
ropieee (https://ropieee.org/)
And how is the AVR connected? USB or Ethernet?
I dont think RooPie includes extension manager. Diet Pi is free ad works on many platforms and includes the extension manager and roon endpoint. See Svenâs earliest post.
Thanksâ good to know
I expected it is included as I included it in rooExtend
You are correct itâs just adjusting its line level output digitally which isnât what you want to be doing to a fixed level input.
Why you giving this advice when op wants to control volume of his amplifier from Roon. Itâs just plain wrong. This wonât help in that at all. It will compound the issue and can lead to having two independent volume controls.
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