TheGadaffiDuck
500+ Head-Fier
I don't know who measured that it takes 30W (in 8 Ohm) to reach 90dB, but I don't think that is anywhere near the reality.
InnerFidelity has measured that HE-6 needs just short of 20 milli-Watts to reach 90dB.
It might not sound as much, but if you compare it to a "normal" headphone at 107dB sensitivity, HE-6 is 1000x (yes, that's a thousand times) more inefficient.
Maybe it was milli-Watts. I dunno. But that's way too much