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

Mac mini costs about the same as microRendu. A good expendable NAS will cost $400-600. Drives like Western digital Red will be cheaper than external drives that attach directly to a Mac. You won't need as expensive cabling. A linear power supply is recommended in either case.
All in, to get a far superior set up it might cost a bit more, but we are talking $1k additional or less, not $10k+. 
Food for thought if you want to test the deeper waters. 
With the Mac, try Audirvana+ software, best soundwise of many I tried. Cheers,
Spencer