I've jut looked for the specs on this pedal and it confirms what I thought - the pedal drops the supply down to 12v for the tube (any more would overheat them), so (assuming it's a 7812 voltage regulator) as long as it's at least 15v and can deliver the required mount of current (300mA) then there's no real benefit to running it at 18v, the 15 one will be fine.
Congrats on a great little unit, and even greater spouse.