For anyone that needs to match nuke default (not the aces ) legacy rec709 viewer →
if you want to completely match nukes behaviour for “linear” EXRs, choose linear/709 as your input tag , this will preseve the gamut as it does convert it from rec709 to rec709 which does nothing which is what nuke does by default 
its mostly useful for graded clips that have been converted to linear from rec709 by nuke , like from a nuke-studio publish .
@Sinan might find this useful as well.