I'm a totally ignoramus with Izotope settings, but fiddling today, I suggest you play around with steepness first, then pre-ringing settings.
i found a steepness at about 40-odd and pre-ringing really low - like 0.05. But I'm not the one to ask.
I have struggled with brightness over the years. So have been quite sensitive to the brighter versions of Audirvana. 1.5.5 was the first 1.5+ version I liked (and preferred it to the previous betas).
But when I first upgraded to 1.6, I didn't like it at all. I reverted back to 1.5.5 and was happy. I was going to post a question on here, but thought I better try again to be sure first. I was surprised to find it no different to 1.5.5.
Moral of the story: try a fresh install .. Even if you already did...
(Use AppCleaner or the like to delete preferences and repairing disk permissions with disk utility)