Hi, I just registered after reading up on all of this for a while. Main purpose was to flatten my new DT990 pro's a bit since they are kind of bright in the high end.
Managed to pinpoint the exact spikes, at least the main ones, and level them out.
For those who have the DT990 pro and wish to slap some EQ on, these are the settings that will eliminate the three prominent spikes in their freq. response :
filter 1 : 150Hz, -3db, Q=1.2
filter 2 : 6450Hz, -3db, Q=4.1
filter 3 : 8875Hz, -7db, Q=5.2
It looks like this on the parametric : http://imgur.com/LAZ0r3D
The way I pinpointed these was first by following this guide, which led me to these big spikes plus some smaller ones.
By adjusting the Q and levels a bit I was able to sum those few little spikes within these three adjustments.
I noticed only the one fairly gentle (read: low Q) boost of about 3-4db between 75Hz and 500Hz for the lows and lowmids.
In the highs above 2K there were these two very big spikes between 5K and 7.6K, and between 8K and 10K.
I used the Frequency Response Data from Goldenears to verify that what I was hearing and finding corresponded to their data, which it did rather perfectly.
My conclusion, personally, is that in the lows and mid lows, there isn't that much of an issue at all,
and representation between say 100 and 500Hz is quite flat although a bit boosted across the band.
But then starting @ about 5-5.5K two huge spikes, which to me explains away their overly brightness...
You can set these filters in f.e. Equalizer APO or any other kind of EQ to have a more 'true' flat response.
I found that by using only these three filters the end-result is even better than when EQ'ing all the little spikes and cuts, ending up with almost 15 cuts.
Try it if you have these Beyers, I thought it a nice idea to provide these for others, since nowhere could I find it for the DT990 pro.
Cheers.