DT990 pro are considered as one of the best headphones under 200$. Some people even compare them to HD800 in some regards(check the french version of "le numerique" and golderears.net review), without having a profound bias. The main reason, I believe, is their semi-open construction(like almost all beyers) which enables them to produce rich, speaker like sound without the pitfalls of entirely closed designs.
 
Nevertheless, they suffer from distant/muffled mids and generally colored/altered sound. Although their distortion and CSD measurements are quite good.
 
So, I decided to do a small eq experiment in order to discover if their "inferiority" is technical or subjective(improvable through eq).
 
Here are my proposed filters for you to try:
 
Filter Settings file
Room EQ V5,00
Dated:
Notes:
Equaliser: Generic
Average DT990 pro
Filter  1: ON  PK       Fc     403 Hz  Gain  -6,7 dB  Q  1,00
Filter  2: OFF  PK       Fc   4.654 Hz  Gain  18,0 dB  Q  3,04
Filter  3: ON  PK       Fc   1.527 Hz  Gain   5,9 dB  Q  1,03
Filter  4: ON  PK       Fc   7.025 Hz  Gain  -6,7 dB  Q  1,79
Filter  5: ON  PK       Fc     730 Hz  Gain   9,4 dB  Q  3,06
Filter  6: ON  PK       Fc   2.302 Hz  Gain  -3,8 dB  Q  2,34
Filter  7: ON  PK       Fc     102 Hz  Gain  -3,1 dB  Q  2,00
Filter  8: ON  PK       Fc     164 Hz  Gain  -3,0 dB  Q  2,00
Filter  9: ON  PK       Fc    66,0 Hz  Gain  -2,9 dB  Q  2,00
Filter 10: ON  PK       Fc    43,6 Hz  Gain  -2,7 dB  Q  2,00
Filter 11: ON  PK       Fc   5.159 Hz  Gain  -8,2 dB  Q  6,39
Filter 12: ON  PK       Fc     953 Hz  Gain   4,1 dB  Q  3,45
Filter 13: ON  PK       Fc     456 Hz  Gain   4,5 dB  Q  4,97
Filter 14: ON  PK       Fc   5.695 Hz  Gain  -4,4 dB  Q  4,89
Filter 15: ON  PK       Fc   3.959 Hz  Gain  -4,9 dB  Q  5,45
Filter 16: ON  PK       Fc    31,2 Hz  Gain  -2,1 dB  Q  2,48
Filter 17: ON  PK       Fc     648 Hz  Gain   3,3 dB  Q  4,69
Filter 18: ON  PK       Fc   4.307 Hz  Gain  -4,3 dB  Q 12,57
Filter 19: ON  PK       Fc     578 Hz  Gain  -1,8 dB  Q 13,25
Filter 20: ON  PK       Fc     557 Hz  Gain   0,5 dB  Q  4,95
Filter 21: ON  HS       Fc   6.000 Hz  Gain   3,0 dB
 
Notes:
1. You will need EqualizerAPO(100% free) or other generic EQ.
http://sourceforge.net/projects/equalizerapo/
2. Bass and treble levels can be further adjusted to suite your taste. For example filter 21 was added later.
3. Give your brain some time to adapt. Humans perceive as correct what they are used to.
4. Pads of your dt990 pro must be in relatively good condition(namely good seal).
5. Please tell me your opinion about the quality of the sound(balance, clarity, lack of fatigue etc.) after listening to your favorite or acoustic music(preferably)
 
I will reveal the procedure I followed some time later in order to avoid any influence.