I have had the headphones for some time now and a colleague has had the 990 Pro. The main difference is indeed the open design of the 990s. The sound therefore has more "space" to unfold. The sound is indeed audibly different and more voluminous. The 770s, however, offer better passive noise cancelling. In addition, the 770s let less sound out.
However, I agree with Anton's opinion. It's best to try them out if you can.

Muddy and very accentuated bass region. The midrange is boosted and the treble is somewhat muffled. Otherwise, the "Beyerpeek" is present, as with the DT990. Basically not comparable with the sound of a DT990 even if it is not a reference. It's best to try them out :)