Ok so lets say by some miracle Kun might be more powerful than Nadd.
Where's the evidence that Nadd is more powerful than Kun, hm? You're stating conclusions without building up to them.
Next you're going to say he was more powerful than some of the most powerful sith lords during the golden age which is ludicrous. I'm not a fan boy it's just common sense that the Ancient Sith are unrivaled.
You're doing appeal to majority yet again, for one.
And, once again, you're stating conclusions without building up to them.
Out of all the Force users that came after the Golden Age of the Sith, I'd say Exar Kun comes closest to them. In life, he acquired a trove of Force knowledge from both the Jedi and Sith described, by the narrator, as "more wealth and knowledge than he can ever use." He displays comparable (sometimes greater) on-panel feats to the Ancients. It is shown that he is able to create artifacts like they did (as evidenced by the second amulet that he has on his right hand during the Sith War).
While their narrative description may be greater, it can be argued that the most convincing part of it doesn't even apply to him. The quote in question is that the Ancient's power was "titanic in comparison to later Jedi."
True, it definitely was. But Kun was no longer a Jedi when he reached anywhere near the peak of his power. Ergo, I'm of mind to believe that Kun is able to contest the likes of Kressh or Sadow. I'm very hesitant to say he'd even come close to Ragnos, who is apparently leagues above even those two.
<edit> In fact, the three of them (Kun, Sadow, Kressh) versus Ragnos would be quite a fight. </edit>