I am using an adapter, always have with my DT880s (like I never unscrew it, its lived on them).
I bought a replacement and somehow the bias seems worse now, like the left can is louder and the right can is still softer.
I'll try to get ahold of another good pair of wired cans to test, I'm starting to suspect I need to recable these cans. I can wiggle it with the new screw on adapter and it crackles and sound changes.
I can also plug this exact setup into my DAC, which also has a built in amp and the cans sound level. Which is why I haven't been suspecting cans yet.