The treble response with the ECX is like an inversion of many IEMs, where the treble rolls off a bit more than Harman but there's one massive spike up around 8-12k for the treble to rally around and give recordings the requisite sparkle. It works to balance the tone out, but it makes the treble sound rather homogeneous. In the case of the ECX, the treble response follows the gradual slope of Harman fairly closely, sounding rather open and extended, but with one big notch around 10k to make things sound extra smooth without losing too much of the recording's individual treble characteristics.
Unless there's a lot of info in that 10k dip I'm missing, this might be the best treble I've ever heard, imho.