You can have negative potency, which would result in basically the same as extra tenacity for the enemies. So the order doesn't matter in this scenario.
It does matter however if you would get like with speed a flat amount and a % amount.
(for example if you have 100 speed, 1 ability gives you 10% extra and another ability gives 10 speed extra, if it's first the 10% and then the 10 flat, you'd get 100*1.10 + 10 = 100 + 10 + 10 = 120, if it's first the flat 10 and then the 10% it's (100 + 10) * 1.10 = 121)
I don't know in which order it's calculated in this scenario.