I do not own the Beyerdynamic DT-250. I HAVE compared them, using a cd of my work as test material, directly with the MDR-7506 in a pro audio store which I frequent.
Note: I've never even had the DT-250 in my studio, so this IS NOT a "review". Just some initial impressions. Take it with a grain of salt and do your own listening!
Having said that, in MY (limited) comparison the DT-250 sounded very smooth overall, with a less "hot" midrange and high end than the MDR-V6, and also with the very good bass extension that one would expect with a closed 'phone (though PERHAPS with a little less energy in the 20-40hz octave than the MDR-V6/7506). The DT-250 seemed several db less sensitive in the comparison, requiring a bit more "oomph" from a headphone amp to reach the same level (as the MDR-V6/7506).
That's about all I dare say, considering how limited my exposure to the DT-250 was. I will say I was (very) tempted to pull my charge-card from my wallet, but I already have too damn many headphones!