990 are open-back headphones, 770 are closed. Open back in general have a bigger soundstage, a more "airy" feeling, a more natural and clear sound. But they don't block outside noise like closed-backs, so they're not useful in loud surroundings + they leak your sound out as well. Closed backs generally focus more in bass. If you never used an open-back I can't rly recommend the dt990 as they will possibly sound harsh and flat.