Let me get an FFT. The distortion measurements above were actually combobulated from centered impulse responses which were in turn combobulated from frequency sweeps. This sometimes yields different results.

purrin, so far as subjective performance goes... do you like these way more than the newer dt880s or do those do other things better?

Probably and likely - I don't have new pair on hand to compare. Mid-treble brightness, but very manageable with EQ. Definitely much smoother overall sound. Doesn't seem to "stab" as much or at all, but again, bright sounding. I think the newer ones seem to extend lower.

500Hz @85db.

The link points to headroom not Tyll's plots..

If I recall correctly, HeadRoom actually use (or at least have used) Tyll's gear for the measurements.

The high amount of high order harmonics is probably not normal, it could be bad quality control, or some unknown problem. It is not present for the 32 Ω and balanced 250 Ω versions, but it is shown for the regular 250 and 600 Ω models. It seems to be fairly random.

Dunno, I haven't calibrated my mic/CLIO system in several years. It could be off or maybe I broke it after some recent high SPL testing.

BTW, I was using A-weighted SPL for the HD598 measurements. The HD598 is fairly even-keen and has a very broad bump centered around the upper bass. A-weighted, SPL is likely to be 3-5db louder for such a headphone than non-weighted SPL which I believe Tyll uses. For trebly headphones, such as the SRH1840, the case should be opposite, which it is (my SRH1840 harmonic distortion numbers are slightly lower than IF.)

I'm likely to discontinue use of A-weighed SPL and line up 1kHz at 87db going forward for the harmonic distortion measurements. The SPL/A calibration is just too screwy.

I think the HeadRoom measurement of the DT 880s are messed up with those roughly equal magnitude high harmonics popping up at every 500Hz interval. That data was probably taken when Tyll was still at Headroom as I remember seeing the same data (but with older visualization several years ago.) We measurers all learn quite a bit over time and refine our methods (it's always tough to implement refinements if we want our measurements to be comparable and consistent with each other.) I know Tyll made a clean break when he went off on his own with IF and his current measurements are different from HeadRoom legacy data.

BTW I think your DT-880 measurements are messed up too. You are getting way too low second harmonic for the DT880 driver or any dynamic driver. In my years of experience, I don't recall seeing any driver have a second harmonic at -68db at any reasonable volume. That's nuts. I know Tyll's IF measurement ~0.3% @1kHz is THD+noise, but THD+noise is typically only a little be higher than THD by itself, not a magnitude higher. Subtract the noise, and the THD is likely to be ~0.15 to 0.25%, with the second order distortion being similar to THD (since second order dominates), not your 0.04%.