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I don’t think the Noire X is particularly hard to drive, so just go with something from there.

Gorguts sounds pretty gutsy on the Noire X. I was surprised about the dynamic range despite that these are “small” driver planars. And there’s enough resolution for busier sections to not turn into mush.

Thanks you for the kind words, John! Actually the Noire X should be fine with most portable sources. It’s a low impedance, medium efficiency headphone, so battery powered sources should be fine with it. I didn’t take mine outside a lot because of the 2m long cable. With a shorter cable, it’s a decent, if a bit overkill option.

My remark about the resistive impedance curve was made to indicate that the Noire X, like many other orthodynamic headphones should be impervious to output impedance induced tone shifts. Most dynamic headphones have an impedance peak at the self-resonance frequency (60-ish Hz) and some have rising impedance in the highs due to voice-coil inductance. A non-zero output impedance would form a frequency variant voltage divider and increase the sensitivity of the headphone at the points of increased impedance.

With that said, the Noire X isn’t terribly hard to drive. A 110dB SPL peak requires the amp to source only 55mA or 40mW. Of course, OTLs can balk at the current requirement but a design with say more paralleled output tubes ought to do the trick.

It should not have been a surprise that your OTL tube amplifier could not drive it. You need a powerful SS for this one, and one that deals well with low impendence and low sensitivity headphones (i.e. not a Violectric or SPL Phonitor).