HE6 Frequency Response and CSD Waterfall Plot
I've never really felt that the HE-6 was that
bright. But I did feel it sounded etched. The sharp 5k resonance (see CSD below) combined with the peak at ~10k probably explains why. 5k is the energy/edge/bite area of the spectrum. John Grados also have excess energy at 5k.
But otherwise very very clean. This matches the subjective experience of those who have heard it.