I have compared these two headphones. Please keep in mind that I'm a happy owner of dt's and hp100 I heard were not burned in. I might not be as objective as You would desire
In my impression these are way different headphones. Dt 990 have really really wide soundstage. Hp100 as well akg 550 (which i was comparing at the same time) sounds awfully congested in comparison. Sound in hp100 and 550 is more linear than loudness-oriented dt990. All of these headphones did well when listening to marcus miller. The main difference for me is soundstage.
You can always equalize to dt990 to linear characteristics using foobar. After that they sound like senn hd650 just with full, punchy bass and wider soundstage. If You throw at the metallica S&M it will be hard to believe it is headphones You are listening to. Honestly. These are my favorites.
In a punch line I would say that when it comes dynamics, spectrum reproduction dt990 and hp100 and 550 perform pretty similar, but with proper equalization dt990 crushes them in an aspect of musical show reproduction. The more sounds You throw at them the more obvious it becomes that a close headphone will always fall back in terms of imaging. Almost the same as open headphone should lack bass. But dt990 doesn't lack bass at all. So if the isolation isn't Your main concern and You have inears for subway this shouldn't be a brainer. Any downfall of a dt990 signature can be track-preequalized.
