Monte,
Is this a correct summary for you?
1 Volumio now shows the hifiberry DIGI+ as a selectable sound device (possibly its appears as sndhifiberry) but you can't get Voulmio to mount your NAS share.
2. Runeaudio mounts your NAS share, but you haven't, or cannot, activate your hifiberry DIGI+ i2s sound card.
As you can see from my sig, I have had a RPiB+/HifiDigi+ combo in use for a few months now. When I tested Volumio and Rune, I did'nt have your problems, but I was using a wired connection and being familiar with Linux was able to check things via the command line.
If you currently have Runeaudio loaded on your SD card, I would check if you've gone through the settings, correctly selecting any i2s options. See :
http://www.runeaudio.com/documentation/runeui/settings/ and then go back to the MPD config page on Rune.
If you still fail to get this working on Runeaudio, you'll have to tackle the mount problem on Volumio. You can safely do some checks at the command line on Volumio to help debug the situation, but not alter settings, as Alan warned, because Volumio maintains an internal database of configuration settings.
One common point of error when entering info for a Windows share on Volumio is having a leading slash in the "remote directory name". If your NAS share is for example \\192.168.1.75\Public\Music, then you'd enter Public/Music as the remote directory.