I did the bass mod on my Q701 actually and though it helped to some degree, it somewhat ruined its dynamics in the midrange and it only made the upper treble roll-off all the more noticeable.
Again, note that the comparison was between a 250 ohm Pro (not Premium; in ZombieX's excellent comparison, the 250 ohm Pro is noted as the warmest of the DT 990 line and now I can clearly see why) and my current 600 ohm Premium and, to be honest, it was going off my memory. But I feel very safe in saying for myself that the Premium 600 ohm's midrange, bass and treble are faster, less grainy and more coherent than the 250 ohm Pro. In the world of Sennheiser, it would be almost like going from a HD555 to a HD600, where you get better extension, less grain and more naturalness in the long run. I personally believe it is a misnomer on Beyerdynamic's part for having made the Pro and Premium models share the same model number. I still cannot believe how good the midrange is and I was always craving for more clarity and fluency in the midrange on my 250 ohm Pros and I always was slighty annoyed by the treble, too. Not any more by a long shot!
Edited by Stillhart - 4/15/14 at 6:11pm