Eventide Stompboxes have internal power regulation and have loose specifications from their external power supplies. Essentially, they need between 9V and 12V under load, with some ripple tolerance beyond that. More than of 15V under load may cause damage. Your 20 ohm resistor will give a current draw of 0.5A, which is more that the maximum draw of these pedals, so, unless their filter capacitors are broken and passing too much ripple, I would say that they are probably OK.
That said, we have had some batch problems with power supplies, so if you find a noticeable difference between them, you should contact email@example.com