I would suspect that the device Amir measured used the NuTube to drive an op amp, because the specs of the NuTube itself are abysmal for headphones i.e. very low power of about 1.7 mW and high output impedance. Also, as has been intimated by previous posters, the original PURPOSE of this device was to add tube distortion to musical gear, because many musicians who use amplified instruments love tube distortion. You probably hear these distortions in a lot of the pop, rock and synth music you listen to routinely. The fact that Korg NuTube has been hijacked for audio use wasn't part of the plan.
Another thing for ASR to go after with a hatchet on the Pass DIY site is the love for Pass' giveaway gift: B1 Korg NuTube preamp boards and plans. I built one of these, and I am listening to it right now as a buffer for my Sony MDR Z1 headphones and it certainly sounds nice. Expands the soundstage or something or other. Even with the impedance mismatch (MDR Z1 about 80 ohms, NuTube out put about 170 ohms), which probably alters the frequency response profile as well. A pair of 300 ohm Senn HD580's also sound great, and at least have higher impedance compared to the NuTube.
http://www.firstwatt.com/pdf/art_diy_nutube_preamp.pdf
It is the only DIY thing I have ever built and I was amazed I got it to work. It also has the intended purpose of adding second harmonic distortion to the signal because that's what some listeners like. Pass makes a distinction between positive phase second harmonic distortion and negative phase second harmonic distortion, and has adjustments to swap back and forth to taste. The Pass device has an output impedance of about 170 ohms, reverses phase, and has said power spec of about 1.7 mW, Heh, Heh! It also 'brags' a two percent or so harmonic distortion figure at 1 volt output, with a gain of 16.
The DIY guys love the damned thing, just like they love the ACA amp, so it's horses for courses. I like it, too, but maybe just because I built it and it works.
My baby, my monster!:
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