A regulated 12V is OK.
An unregulated supply (such as a transformer-based wallwart) should be rated at 9V, as it could be higher than this at times.
More than about 15V (even for a short time) could cause damage to the internal circuitry.
Note that any damage caused by the use of a non-Eventide power supply is not covered by the warranty, so best to play safe.
