Oh, dear. Kun as an amateur Sith needed rage to power up his amulet and kill Nadd. LOL.
Exar Kun grew in power over the next year and became canonically more powerful than Nadd. I'm not sure if his "most powerful" quotes consider his trinkets or not. If it does, then they're pretty close (Kun's still ahead) in power, but if not, then Kun + trinkets = Revan gets spanked hard.