I believe the relative potencies at beta receptors is ISO > EPI > NE
Blocking NE reuptake would increase the local concentration of NE however this does not affect the potency of NE at that receptor. Potency refers to the amount of drug needed to produce a certain effect i.e. the less drug required to produce the same effect, the more potent. A greater concentration of NE would produce a larger effect by virtue of there being more around, however the potency has not changed.
EDIT: I thought about your question again and understand your confusion. I'm curious to know the answer to this as well.