One of the biggest differences is how much strong the dynamics are on the DT 1990, especially in the bass. The bass will be a massive improvement over the AKGs, the mids aren't quite as forward but they aren't what I call recessed. Treble is a bit different, little less energy in the lower treble but the treble is also more refined and less fatiguing, I find the DT 1990 a tad darker with balanced pads. They resolve more detail, better clairty, are less fatiguing, more refined, etc. They are on another level. DT 1990 Soundstage wise a bit smaller and more intimate with the balanced pads. But with the analytical pads I find the soundstage notably larger. The DT 1990's sound is very system dependent you can get some completely different results, it goes from dark to bright, narrow to very wide, etc. depending on the system. The dynamics consistently stay quite strong I noticed. A simple tube change can essentially double the soundstage size and change the sound character quite notably on the DT 1990, never quite experience the K712 transform from system to system near as much as the DT 1990 can.
Unlike other K7 series, the K712 isn't all that wide sounding, but the K712 gains more focus, cohesiveness, and sound more natural than any other K7 AKG. I would actually say the vocal/instrument tonality of the DT 1990 is pretty similar but better than the K712 just as the K712 is pretty similar to the HD 600, just the DT 1990 pretty much does everything better and is more dynamic and lively.
Note: The K712 is the only K7 AKG I actually liked and I've heard every single variant of the K7 family.