I have KK, Sultan & Ragnar here at the same time. Metal is not my thing, sorry.
All 3 are equally a joy to listen to, I guess I just like the Noble house sound, I can clearly hear elements of it in all 3. I could actually live with any one of the three models quite happily, ears have a funny way of adjusting.
The KK has a strong bass and great highs, Sultan to my ears has more bass but a little wooly when compared to KK, more mid bass and mid range, but the treble is a little less full than KK.
Ragnar to my ears has a more refined and better controlled bass, clearer mids and incredible highs.
I guess the best way I can describe the difference between KK and Ragnar is your stereo is playing loudly in a room with a closed door as you approach the room, you can hear the music reasonably clearly, opening the door the detail and clarity and sound stage open up.
I get a sense of extreme detail in every aspect of the FR with Ragnar, sub bass is never boomy, but the flutter of the woofer is very apparent, the mids are uncontaminated with bass bleed and the highs have body and texture.
Take everything I’ve said with a grain of salt, as I’ve also found source and media files, fit and insertion depth all play a huge factor in getting the maximum musicality from any of these IEMs.
But the first day I received my Ragnar I described them as incredible. They still remain so and easily my top choice from what I have to hand.