That's a rather easy comparison. The DT 1990 is similar to the DT 1770 but better in pretty much all ways. The DT 1990 is more even in it's FR response, smidgen brighter, and the entire sound has more depth and body than the DT 1770, so the DT 1990 sounds fuller and richer than the DT 1770 overall to my ears. I see no need for the DT 1770 over the DT 1990 unless you need isolation imho. Plus the DT 1990 has deeper more comfortable pads than the DT 1770 making it notably more comfortable to my ears. The euphony on the DT 1770/1990 doesn't necessarily come from lushness, rather they have a sweet sounding midrange with a nice dynamic attack which makes them quite an addicting and engaging listen. The HD 650 is more lush and laid-back in it's sound but still has some of that sweetness to it's mids, just not as much as the DT 1770/1990 does. The Beyers are more aggressive and brighter than the HD 650, but I find the DT 1990 more dark and smooth with a lot more body than the K712. Listening to the DT 1770 will give you a glimpse into the DT 1990, if you really like the DT 1770 but want more or had a minor niggle about the sound, go for the DT 1990.
I have only owned a few headphones I felt no desire to EQ, use very select system matching, or modding as they are great as is. They are the HD 650, the HD 600, the DT 150, and the DT 1990. I feel these four nail the all-rounder aspect with essentially no fatigue. I don't find any to have any serious sonic faults that can hurt the listening experience. Also while the DT 1990 may be a bit brighter than the HD 650 I also find it least fatiguing headphone I've owned or auditioned at length, including the Focal Elear.