I wrote to MOTU to clarify and I'm just posting the responce so everyone else will know:
On page 6 of the users guide, the back panel reference states that the device will take between 10 and 24 volts. Later on page 19 it states that the unit takes between 8 and 18 volts with 12 watts needed.
The reply is thus:
Please follow the recommendations on page 19 using 8-18 volt, 12 watt DC power supplies.