There might have been a little extra prominence in the bass of the 250 ohm version, but we're not talking huge amounts.
Newer 250 ohm DT990s are improved from the ones that were being sold at the start of this thread a few years ago. I've been able to directly compare a DT990 250ohm from 2007 and one from this year, and the difference was surprising. The treble wasn't as piercing and hot in the newer version, the bass had a little more power. Then I exchanged the 2010 250ohm phones for the 600ohm version, and the difference wasn't nearly as striking. Perhaps less distortion, cleaner sounding, but without direct comparisons, such very subtle differences would be hard to pinpoint with any accuracy. I'll simply say that I don't think the differences between the 600ohm and 250ohm versions that you read about in threads dating back to 2007 or earlier are very representative of what you'll see today. Newer 250ohm DT990s are simply improved from older ones.