hey guys i have had this same problem and there was a fix for the original raspfi for usb dac output that should fix our problems. here is the link i have found raspyfi.com/raspberry-pi-usb-audio-fix/
i am installing it now to see if it fixes the problem but i have 3 usb dacs that have the same distortion so that tells me there is some packet loss.
list of dacs
3.ud110v2 by stoner acoustics
ok here is update on the fix
i ran from the pi
sudo apt-get update (let it update)
sudo apt-get install git-core
sudo apt-get instal binutils
sudo BRANCH=fiq_split rpi-update
then walk away for a few heat you up a cup of coffee or get a beer it will take about 10 min or so then restart your pi i didnt edit the mpd.conf
i just left it alone and it has no more stuttering!