They were (all 6 together) defeated and captured by a Jaric Kaedan in his prime and a small republic strikeforce. Kaedan was considered a living weapon of the jedi order at the time and a master of his own style of Form VII: Juyo'Kos. So if the living weapon (arguably the most powerful jedi of the GGW at the time) needs a bit of help to win against the Dread Masters well you get the idea.
And before you say you kill Jaric Kaedan in an Ilum world quest but it takes 8-16ppl to kill 1 dread master hears something to chew on. Its been over 30yrs since Kaedan and a small republic strikeforce beat all 6 dread masters together, (They are at their absolute peak of their power when all 6 are together but thats beside the point) Kaedan at this point is an old man whos far past his prime when you encounter him on Ilum.
Also game mechanics dont work well in determining ones power. There is no way possible that Jarg and Sorno are more powerful than Darth Malgus, its just game mechanics that say otherwise.
He was against the dread masters, not one but 6. Even with help of troops. The dread masters were a force to be recon with but far from invincible. I doubt that jaric kaedan would last 1 minute against the emperor.
Him being the most powerfull jedi also debatable. But even considering that, i cant see how the dread masters are more powerfull then the emperor. wich was the point.