Hi,
sorry to dig this out, but what was your conclusion in the end?
I am having a similar issue with my DT-990 Edition (250 Ohm Version). In fact, if i push the signal enough for it to be nice and loud, there is already some clipping (not the good kind). Just opened Reaper to check my levels and there it also shows clipping. Therefore i reduced the volume back down so it shows around 0 dB in Reaper but could be louder for the heaphones really....
Thanks!