This has been an educational journey!
With the investment of about $500 and 8 hours of my life, I built a fanless NUC 10 i3 in an Akasa case (a huge heat sink,) installed Roon ROCK Core software and hooked up my ripped CD library (from the Innuos Zen MkIII server) in a 1TB SSD attached to the NUC. (No upscale power supply or network cables.) (It only took 8 hours because of a couple 3-hour rabbit chases due to lack of specificity in Roon's instructions in places.)
Then I switched the Zen into Player (only) mode and WOW! Best SQ I have ever had in my system by a wide margin. Even my wife noticed.
The change I noticed first was the amount of "air" and detail in tracks I had listened to dozens of times, closely followed by more accurate tone in instruments and vocals. It's like I could hear the boundaries of the room where the recording was made. The decay of cymbals lingered much longer. A tenor sax really sounded like a tenor sax.
This was the type of improvement I was expecting from moving up to a better server streamer.
I also tried hooking the NUC via USB (Audioquest Diamond) directly to my Yggy DAC. That was also better than my original arrangement but it didn't have the magic of having the Zen player in the loop. (That could be due to the generic power supply on the NUC.)
@sns is also absolutely correct that the very best SQ comes with turning off the (very handy) volume control, volume leveling, DSP EQ etc., no matter which configuration you run. I had not previously discovered that. Now I have to get used to walking over to my 300B integrated tube amp a lot more frequently to adjust the volume!
I still plan to upgrade the server/streamer, but will probably stick with that magical NUC running Roon ROCK on my network. I should have the new server streamers in to to compare in another month.