Just popping in here again. A long while I posted about doing this with my DT880. Since then I've refined my methodology (or, more precisely, did what the tutorial actually said and used the pink noise instead of just sine-sweeping) and the results were quite different. Basically what I did wrong before was lower the "baseline" along with trying to suss out the peaks. Therefore, though I finally got the peaks sorted, everything around them was too low as well. This, I suppose, is why the tutorial recommends the pink noise, so as you can make the notches just as wide and deep as they need to be to eliminate the peaks and nothing else.
Some folks wanted to see my results before, so here's the updated curve:
Filter 1: 5827.6 Hz | -6.0 dB | 0.3 Oct BW
*Filter 2: 7577.8 Hz | -6.0 dB | 0.5 Oct BW
Filter 3: 10920.0 Hz | -6.0 dB | 0.5 Oct BW
Filter 4: 13879.0 Hz | -5.0 dB | 0.3 Oct BW
*This peak will almost certainly occur in a different place for different people, as I believe it's related to my own ear resonance.
What's really interesting is that it matches up very well with the following, even though I used nothing but my ears to make my curve:
These are the raw frequency response curves for the DT880 and the DT990. My curve lines up almost perfectly with the one for the DT990, leading me to believe that HeadRoom's plots for the DT880 are off, or else my sample is more DT990-like in its treble. Maybe this is why so many people have different impressions of the DT880--could there perhaps be some DT990-esque DT880s floating around out there? (FTR, I have the 250 ohm version, and both these plots are for the 250 ohm variants of their respective models).
Anyway, I trust my ears, not the HeadRoom plot, and my ears say that there are four ~6 dB peaks of varying widths. The second peak in my plot, which occurs above the 5-6kHz one (and which is absent from the HR graph), seems to exist in every headphone I test, so I think it's some sort of resonance peak (it's usually around 7.4kHz, which is double the 3.7kHz presence peak). Also, my third peak is centered higher up than in the above plot. Looking through some of the DT880 curves others have posted, I see I'm not alone in my final result. Many folks have four peaks, usually of similar amplitude and centered in similar places.
But the big question everybody probably wants to know is, what's it sound like EQ'ed up? Some of that special airiness that the DT880 is known for disappears, but with it goes most of the harshness and sibilance, and the overall balance is much more natural to my ears. Detail, of course, is somewhat lessened but not to a terrible extent. As far as I'm concerned, that detail was never there to begin with if it needed the extra treble to bring it out. Overall, these are still airy, quick, and resolving with the EQ, except now they don't bite my ears off anymore. The biggest revelation, actually, is the midrange--it really perks up (which it would have to, because, as I've argued before, it's not "recessed" between the treble and the somewhat light bass on these, but rather overshadowed by the high end).
Overall, I highly recommend following this method. I offer my curve just to illustrate that there's not a whole lot that needs to be done to a DT880, apparently, so it's a good candidate for this type of adjustment. Don't copy my result verbatim; your pair of DT880s is likely different (more like the HeadRoom ones perhaps?), and so are your ears.