Best Mods for a Mac-mini used as a Music Server

Dave, 
Probably not what you are hoping to hear, but...
I used one for years, used all the top software packages, usb cables ranging from AQ to Black Cat to the $1K Lightspeed, tried all kinds of tweak from better power supplies, ifi cable splitting filters, decrapifiers, etc. Had Lacie hard drives and tried SS drives too. 
The best thing I did was replace the mac with a Sonore microRendu and a NAS. It wasn't even close. 
There are plenty of threads that explain the advantages of my solution and plenty of similar approaches. Chris Connaker's online 2part review of the microRendu is a great place to quickly get up the learning curve. 
If it's not too late and you aren't emotionally "all-in", consider it...Cheers,
Spencer

Here is an article about going the Mac Mini route:

http://www.theabsolutesound.com/articles/turning-a-mac-into-a-music-server/

The power supply is a big deal. If you can get a linear supply, do so. Regardless, put it on a separate conditioner outside your main power conditioner. The inexpensive Furman or PS Audio strips work really well, with LiFT and SMP, like this one:

https://amzn.to/2XsjjEY

It adds filtering between your Mac and your audio. Another similar but not quite as good alternative is the PS Audio power strip, and they run 2x the price.

Best,

E