I asked Thomann a long time ago for a straight line and 80 ohm version. Finally she appeared.
In my opinion, 80 ohms is the optimal impedance.
After a while I will add a review.
I tested a very large number of headphones. I had a lot of things at home for a long time.
In my opinion, the DT990 is one of the only low cost open headphones that can reproduce bass.
When the 80-ohm version with a straight cable appeared, I immediately bought it.
HD660 will be distorted by the speaker. But I haven't tried 58X.
The HD560S will also distort the speaker.
About AKG - it seems that this company does not like bass)
But, I did not have time to try 612 and 712.
For a long time I had T60RP, this is an isodynamic driver, it can play very loudly, respectively, bass and sub-bass are reproduced very well, on a large scale, but all other frequencies seem to sound narrower than bass.
The scene in the 990 sounds solid, natural, wide and deep.
And unfortunately Fostex has poor quality control, probably like many others.
I also remember the old Shure 840, I sold them because of the weight) Although they were closed, they were very cool. I really liked their sound.
Constructive disadvantage 990 - If you have a small head, watch how the headband presses the ear pads to your head.
Keeping the speaker away from the ear affects the sound badly.
If there is more pressure on one side, this will also affect the sound.
ps in headphones, the most important thing is the ear pads, they will most affect the sound)
The sound signature is of course harsh, if they need an equalizer to listen to music, but they tolerate it well.
I dismantled the headphones, the cups inside are the same, with the same number of holes, they close tightly. this is to protect against channel imbalances such as in the DT250/(80 or 250 ohm).
I think it's worth having both 990 and 770, these are different headphones for different purposes.
I like both.