Hi Dave,
The Pi is running Rune v0.3-beta and powers up on its own thru its micro plug
With the Sabre DAC installed you do not need an extra power supply for your Pi, because it will get power from the DAC.
Please see the
documentation
You need a momentary or non-latching switch (push to make) to activate the ATX like power supply.
A 6 - 7V external power adapter should work.
To get all the comfort from the DAC you also need a LED and a few scripts installed on Rune.
Please first try to get it working with a simple button.
After that i will guide you through the rest.
Frank