I do wonder if its a driver matching problem as all DT250's seem to be doing the same thing.
There is a large PCB inside one of the cups (for the mic. acc.) which may account for the differences between L and R in the bass and lower mids area.
Perhaps spoony could remove it and listen again ?
Tonally they (I heard and measured spoony's DT250 when it was still DF's) are correct to me.
My only gripes are clamping force and the refinement of the treble.
One has to include the price range it is in (quite low), and the fact that it isn't (marketted) as a hifi phone but intended for monitoring not critical listening.
They have other headphones for that

It didn't seem rolled away anywhere to me.
Didn't listen to it for a long time though, perhaps other issues may have creeped up on me.
never bought one myself though.