Although there is a tiny bit of slack (12V rated power supplies don’t always output 12V) it is really really really not recommended to use a different-voltage power supply. (18V rated power supplies don’t always output 18V either)
On a more practical note, the Dunlop DC-Brick is not able to provide enough current to power up the MOD Duo.
We recommend using only the provided power supply, since it is not only a guaranteed way to not destroy the Duo, it is also very quiet. Whereas other power supplies might cause some extra noise on the output.