I got the NAD HP50 too, smoothest headphone I own, and when solderdude measured it on his website it showed extremely low distortion (https://diyaudioheaven.wordpress.com/headphones/measurements/nad-viso-hp50/) .....but NAD HP50 is also smooth in the treble in as much as lacks a bit of the upper treble and doesn't have crazy peaks up there (in terms of measurements on GRAS dummy heads, and also in most people's listening impressions)......it's a really great headphone that I like more than my Senn HD600, but NAD HP50 doesn't have the speaker-like quality of my AKG K702 (so that's my favourite......all headphones EQ'd to Harman Curve though).Well I got a few Harman tuned headphones and IEMs in the past.
K540, Philips SHE3590 IEMs, Blon BL-03 IEMs, NAD Viso HP50, Ostry KC06a.
They pretty much all sounded like each other in terms of fr response, some had more refinement than others, but the balance was the same, as I described in my previous post. Except for the NAD, which had a smooth upper-treble, but still was too emphasized in the upper-mids.
It could be that my ears are different from average, I don’t know. But, I also have the HD650, and even though it’s upper-mids are slightly emphasized, it’s slight, and not intrusive at all due to the low distortion.
You need very low distortion to pull such a fr curve off correctly in my experience. In practise, most transducers can’t do that.
Harman Curve isn't really a difficult to "pull off" like you say for most headphones when you EQ them to the Harman Curve....although the bass is generally the hardest part for them to completely match, with closed backed headphones most likely to do so....in fact the NAD HP50 we both mentioned, that does Harman Curve bass to perfection (I think) due to it's flat extension of the bass with no roll off down to 20Hz & below, and it also has super low distortion too. (In fact it makes me want to get some subs for my in room speakers!)
Having said that, quite often headphones have issues in the treble regions with crazy dips & peaks which makes them very difficult to EQ sucessfully to the Harman Curve. But, in general, it doesn't take much to "pull off" a Harman Curve for a headphone, or at least something close to it.
EQ'ing different headphones to the Harman Curve will produce different results in how they sound, general tonality will be roughly similar, but there's always some discernable difference, and for me it was choosing the best sounding headphone once it was EQ'd to the Harman Curve (AKG K702) because I knew Harman EQ improved every headphone I have....but it'll be different for different people in terms of whether you even like the Headphone Harman Curve & also subsequently which headphone suits you best once it has been EQ'd to the Headphone Harman Curve. If none of your headphones sound better or right after being EQ'd to the Headphone Harman Curve, then I think you can conclude you really don't fit the average anatomy mould, at which point it'd be best to either tweak different parts of the frequency curve around Headphone Harman Curve to work out which bits need to be changed, or instead use a completely different EQ process unrelated to measurements & the Headphone Harman Curve.