I currently own both. The HD598 can sound a bit thin compared to the D2000, as the Denon has wonderfully deep, powerful bass. However, what bass the HD598 does have is detailed, tight, and textured. The D2000 can, at times, be just a tad too boomy and muddy. I wouldn't call either particularly laid-back headphones. The HD598s have a better, bigger soundstage than the D2000, though the D2000 is really quite good for a closed headphone. It doesn't sound claustrophobic like many closed headphones do. The HD598 also excels in the midrange compared to the D2000, which is slightly recessed in some midrange areas.
Both are comfortable. The HD598 has more clamping force, but is lighter on the head.
Really, they're both great do-it-all headphones. I think the HD598 offers a more balanced, smooth/natural sound than the D2000, but I've also come to love the midrange on the HD598. Prior to owning the HD598, I didn't think the D2000 had recessed mids. Still, I'd use my D2000 for bass-heavy music any day over the HD598. In other words, the D2000 has more of a U shaped frequency response curve, while the HD598 is closer to an upside down U shape.
You can't go wrong with either. If you have any other specific questions, feel free to PM me.