Which pads did you use for these measurements? Wondering how much of a difference they would make, and in what way.

This. The velour pads make a considerable all-positive impact on the sound compared to the stock pleathers, everyone over at the HE400 threads seem to agree. Were these measurements done with velours? The FR looks quite different from the Innerfidelity / headroom measurements, namely the lack of huge 1kHz hump, so I'm guessing you did use the velours.

Nevertheless purrin, very interesting measurements indeed, there seem to be a lot that these do great with and some small issues as well. The treble tuning is pretty clever IMO, think of it this way - if the ~5kHz range is tuned to regular levels which the 14kHz+ stays where it is, then the overall treble might sound overly bright rather than this "dark but with good sparkle" signature. Very cool that this has some of the cleanest bass too.



Also, I wonder if the high distortion is a trade-off for the high efficiency?

Why?  Are these easier to drive than the LCD-2? 

LOL. I forgot to mention they were slightly modified. I used the stock pads, but put a thin layer of polyfill between the drivers and the external grill. This essentially raises the depressed area 1db, lowers the bass area 1db, but manages to leave the treble alone.

The speculation that higher distortion was a result of increased efficiency could be true.

Could you try out the velour pads and perhaps do a measurement with that too? If you still have any of the other Hifiman headphones you should have some compatible velours lying around that can be popped on these. It'd be great to have some objective confirmation (or debunking) of the sonic effects of the velours on the HE400s.

The polyfill mod sounds VERY interesting though.

I'm just not registering the significant valley centered around 5kHz when I do my SineGen sweeps. In fact it sounds just as loud at 5000 Hz compared to 1000 and 8000 Hz. The sharp "artifact" dips are still present though. Perhaps the velour pads raise that upper midrange valley? Or maybe my pair just has a better tonal balance than your pair purrin.

Are you doing them by ear? Because I don't totally trust sine sweeps (because of distortion)...

Yep, and I see. I don't know how to do the white noise thing you described though.

The other thing is that the dip around ~5kHz is an artifact. Check the CSD. There's actually some minor ringing going on there.

Totally missed that; so a lot of these sharp dips are just artifacts caused by ringing then, it seems rarely the case with your measurements here that a sharp dip is actually a sharp dip, something ugly always seems to cover it up.

Nevertheless, again, please do take a quick measurement of these with velour pads when you have the time.