Pros: Total clraity tone、ultra transient, froward,focus,but relex present
Cons: Very pickly (about amps), Headband permeant sweat,sensitive about environment humidity
I would make it short.
1.This headphone has strong resonance in 80~100Hz, you would need a very strong amp to overcome the emf, or would got a notable unnatural bump in upper-bass.
2.About the infamous recessed mid- I guess it's a slight,but critical QC fault:
Inside leather bag, the humidity is been lower than normal condition by desiccant,makes the damping coating on the diaphragm lose it's function, just need sometime to got proper work.
3. Compare DT880, what you can got extra from DT990 : Highs with natural brilliance , richness low-mid and bass. not "flat" as DT880, but makes everything more believable and vivid , a step up transparency,and the most important thing is :a unusual believable holographic field.
4.Tone character is close the "Yin" side, upper mid and lower highs has the typical Germany-sound feature,as Burmester,Lindemann and ASR, in short, it's a kind of engry of highs, Sweet, clarity, but need proper tune to avoid overly bright or "hot". Especially the upper mid ,this band is always the Achilles' heel of full-range driver.
5.In some best conditions, DT990 would got a unique perspective:It's a head stage no stage, a sound field really a field in front of, Kind hard to describe, a feeling of realism,not only "disappear", but has a stunned ability of rebuild whole scene, A Stage of The Arts class performance.
Some of best headphone can rebuild the scene as well. But really different thing is: The scene DT990 rebuild didn't has a notable "Vanishing point".
Headphone's "King of the Bokeh" ? Maybe, really stunned beautiful sound-scape.
6.Finally summary : One of the best/underrate headphone under $500.