I listened to some frequency sweeps as soon as I found tips which seal properly, and found that the treble sounds distinctly quieter above about 4500Hz, with a little spike at 6kHz which doesn't seem to go louder than the level below 4.5kHz. But below 4.5K it sounds very flat to me all the way down.
The difference between then and 15 hours of burn in, is that there were also some little jitters of volume through the higher end of the sweep initially which seem to have all but gone.
I'd be curious to know if this impression of a frequency sweep matches that of others, or to what extent it's just the combination of my ears, with these tips and this particular pair of B3s.
I must say that if the sweeps give an impression of something not great, it's far from my experience of these iems.
Subjectively these are a little more modest sounding than my Fostex TE-05s which are flatter in the highs, a bit raised even, and more prominent in the lower bass without having a discernible bump - the TE-05s sound a bit hyper-real, in the way lots of good modern hifi does, there is a slightly emphatic artifice to this effect even though it also presents the timbre of instruments very well. The B3s I think sound more natural, but the modesty of sounding subjectively not better that real, but fairly real, is great for me, it's what I want.