Is using an Apple Computer a weak link...?

Craig - All very good questions…!

I'm curious WHY you want to go from The Aurender A10 to a Desktop unless you can't use it as a dedicated streamer. But I'll stick around and maybe learn some more.

I started streaming using a MacBook Pro running Audirvana. I briefly tried a Node 2i but ended up returning it. The SQ of the Node was on par but I preferred the UI of Audirvana. Still not satisfied I purchased a Bel Canto eOne Stream. I much preferred the SQ of the Bel Canto over the MacBook, adding a W4S DAC to the signal chain made things even better. I firmly believe that a good dedicated streamer will outperform a PC.

I am under the impression that the "effect" of a computer on sound quality depends on the software used to send the music file to the DAC.

The major piece you’re missing is noise.  Computers, especially if they’re used for things other than just playing music, infuse noise into the signal that good dedicated streamers largely avoid. 

Dedicated streamers are just computers that have been optimized for audio performance.   Compared to a bog standard PC they tend to have better parts, better power supplies, and inject less noise. Programs such as Audirvana do help the sound from a computer.
The noise issue is real, but it isn’t like like vinyl, where it sounds like frying bacon.  It is mainly the depth of the black background.  Compared to better equipment, the music just doesn’t seem to emerge from nothingness, and therefore seems a little less tactile.  You don’t really notice it until you compare with better equipment.