30 seconds with the Beats by Dre Solo HD mark I taught me one thing: bass farts exist.

It was a demo. If it was my property I would have used that headphone for target practice in "play darts with garbage bins."

Some headphone sound so bad that the ears and brains say no. Never ignore that signal...

you need special software that has a sliding notch filter in the measurent path that runs along with the sweep frequency.
Latency is of importance here for accuracy.

I don't see the need though and REW and ARTA can also give you THD plots.

Well first you need to determine the THD.
Usually this is slightly above the highest harmonic contents you see in the plot.
Then you have to look at each individual frequency (say 100Hz for instance) and determine the distance between the fundamental (upper trace) and the highest harmonic peak and calculate the percentage from the dB scale: http://www.sengpielaudio.com/calculator-thd.htm
the distance is 55dB in this case which works out at 0.18% for 100Hz.
AFAIK Marv measures at around 90dB SPL so this should be close the blue/red traces.

Tyll's plot shows the THD somewhere between 0.1% and 0.2%
Ignore Tyll's peaks at 20Hz, 200Hz and 2kHz.

Personally I think the plots showing 2nd, 3rd and 4th harmonics to be more telling/informative than the THD.

1) I've given up on calibrating with ARTA. One day I will figure it out.
2) I take distortion measurements where the 0dbFS line in ARTA = 100dbSPL. You'll have to get a SPL calibration with your mic. I did my calibration via a second system / CLIO. In general, I try to line up the lower mids/mids to about ~90dbSPL, which is extremely loud, but I think it gives a clearer measurement result that more easily correlates to what we hear.
3) I use an Objective 2. It measures very well and has good power and low enough output Z. It is also cheap. Just don't overload the input because the pot after the voltage gain stage is a retarded design.
4) All the lines matter more or less. Overall high distortion is never good, so looking at the peaks doesn't necessarily make sense. The different lines represent different orders of harmonic distortion from 2nd to 4th order. How these kinds of distortions sound like is beyond the scope of this thread or website. We are still learning. Listening and looking at measurements to see how they correlate. What also matters is how headphone A compares to headphone B. Measuring without listening is stupid. Some measurements correlate to experience more than others. That being said, some people say even order or 2nd order distortion is more euphonic than odd or 3rd order. Personally, I think that is bullshit. 2nd order distortion sounds just as shitty as 3rd order, just a different kind of shitty.