A) It's a mighty assumption to just claim Nadd is on par with Exar Kun. There's been no primary source material written on his life, it is extremely difficult to say.
You've said it yourself, that the amulet can destroy spirits. He certainly didn't look like he was going to die without being attacked by the amulet. He did lord over a prodigious Kun, even if it wasn't a dark side one. His feats even as a spirit were mighty impressive, moreso than Kun's feats as a spirit, even though Kun's spirit did linger for much longer.
Also considering Kun did get a good portion of his knowledge from him, and that Nadd's personal accomplishments included conquering warlike Onderon and destroying the forces of its moon, it's more than reasonable to assume that he has plenty of power of his own. Can we say it is definitive? No, but we can't say with practically any of the ancient Sith. I would place him right with Kun, based on what I have seen and the logical inferences thereof.