How to set up a Mac mini for music only?

If you don't already have a Mini, I recommend getting an Oct 2009 Mini on ebay with keyboard and mouse for around $375 that can be powered from DC supply. This was the last generation that could be DC powered without having to do mods to it. You can put a SSD and 16gigs in this one for about $200.

Steve N.
Empirical Audio

From a hardware perspective:

I'd start with upping the RAM to as large as possible. With the 2014 Minis, this means getting the factory upgrade as the RAM is no longer user replaceable. Not an issue with the 2012 and earlier models.

I'd go with an SSD option (not spinning HDDs or hybrid/Fusion drives).

You can opt for a linear PSU transplant later on - the switch mode PSU inside is replaced with a cable or DC filter board and you'd need an external PSU.

Unlike Steve, I don't find the older Minis a good match - I've found in my PC builds that a faster i7 QuadCore sounded better than my Atom based servers so I'd say go for the fastest CPU you can get.

From a software perspective - it USED to be difficult. You needed to use sudo/terminal commands to disable a lot of the background processes.

But here's the good news - Amarra now includes a Wizard based optimiser that does this automatically for you. It disables Spotlight, enables Finder Quit, Spaces etc. The same Wizard allows you to re-enable the services if you wish.

Audirvana Plus also provides something similar but I think it is not as extensive as Amarra's. On the other hand, it is a lot simpler to use (just a check box and a restart).

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USB-SPDIF devices

I know the Offramp is highly regarded but up till the previous model, it still used an M2Tech USB converter IIRC which had two major downsides.
a. it wasn't USB Audio Class 2.0 compliant which meant it needed custom drivers from M2Tech for OS X. OS X from 10.6.8 and up had not needed drivers for USB Audio Class 2.0 drivers - they just worked when you plugged it in.

Mavericks (the previous version of OS X) broke compatibility with M2Tech's drivers for 10.8). And it took over 6 months for a new version to arrive. I'd just go for an Audio Class 2.0 compliant converter for the hassle-free experience. I don't know if M2Tech's driver works with Yosemite but I know Mytek (which also needed its own driver for Mavericks) doesn't work in Yosemite now either.

b. A consequence of USB Audio Class 2.0 compliance is that most apps like Audirvana Plus, BitPerfect and Pure Music can enable Integer modes which IMHO brings the performance up a fair notch.

If you have a Bryston BUC-1, I'd keep it. It is quite a well regarded converter as well.

Doggie - these drivers are not the problem that you say they are. Takes a minute to install one and then you are done. The reliability and sound quality of the M2Tech solution makes it worth a minute of your time.

The XMOS interface does not need drivers for Mac, but does for PC, so it isn't much better. I have mine running and its a lot more sensitive to USB cables than the M2tech. There are always tradeoffs. My M2Tech-based interface is solid and works great with all 5m USB cables. Beats every XMOS interface, even in integer mode.

If you want the best sounding USB interface, its Empirical Audio. It wins every shootout, including the latest:
http://forum.audiogon.com/cgi-bin/fr.pl?ddgtl&1398132150&&&/Absolute-top-tier-DAC-for-standard-res-R

Steve N.
Empirical Audio

"Unlike Steve, I don't find the older Minis a good match - I've found in my PC builds that a faster i7 QuadCore sounded better than my Atom based servers so I'd say go for the fastest CPU you can get."

I don't understand why that would be. Playing music on a computer can't be all that resource intensive for a modern PC. CD's came out in 1982. If you rip one to an equivalent lossless file, why would you need all that processing power?

Disclosures plz