Yes the HD598's imaging is absolutely superb... even better than its higher number counterparts (HD600/ 650). Compare that to a Q701 which very poorly integrates left and right channel without having a focused center - a lack of proper 3D rendering.
The DT990, I have never heard myself. But it is a very v-shaped sound that it produces. The treble is unnatural on that phone and will need serious EQ to compensate. I've heard the DT990's bass is sometimes a bit too much, and at times flabby. But this is all relative. Another person could say it is just the right amount and tight, depending on what they've previously heard. Certainly this is a very different phone from the 598.
The HD598 has more bass than my "bass-heavy earphones" simply because it extends better. If you have never used a decent headphone before, you may feel that the HD598 has quite a bit of bass to offer. If not, a psychological "burn-in" will take place and you will get used to the 598's bass.