You should look at how it compares with DT880 250 ohm 2005 edition. It's on par with it. But my DT880 still under amped right now. It still won't go too far even if properly amp since it's quite sensitive.

Not sure yet still haven't open it. Now i'm still having the wow factor with the HD668B even straight out of the box. It's on par with Dt880 in performance but they take a different approach in their sound signature. Dt880 still will win if properly driven but i don't think it would be too far.