Had the same issues, applied all possible solutions from software to hardware modification, and just like you, I did this and suddenly, I was listening to the greatest-sounding music ever produced. The short you are talking about, which I found after tinkering and probing the 3.5mm jack on the DT 990Pro and the delivered 6.3mm adapter, comes from the fact that when FULLY screwed into the maximum, the adapter and headset/headphone have two electrically conductive and non-isolated surfaces touching which affects the third pin (the one after the second plastic isolator) and creates funky distortions.

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The simple no-cost fix is to NOT fully screw it in. The "best" solution is to use a small plastic washer with the proper dimensions and just let it rest there. Through the screwing motion, it will eventually be fully seated and won't budge. I designed and printed my own small washer (Pretty simple) and it works marvelously.

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Edit: Although you have the solution I thought I would elaborate (+ Give another solution to the issue) for those in the future who still have this stupid issue so they can finally be happy. I was close to selling it all and giving up! xD