I haven't owned the 598s, but from what I've read the consensus is that the 598s are very flat with a good soundstage, however I do own the AKG Q701s which, from what I've read, are described as also being very flat and having a good soundstage, and an upgrade over the 598s. Having both the AD900X and Q701, I think they are very comparable amped. An amp is required with the Q701 as they don't get loud enough otherwise, while the AD900X still sounds great without an amp.
I've heard that the Q701s have a larger soundstage than the 598s, and the AD900Xs have a larger soundstage than the Q701s. The AD900X's mids and highs are more forward than the Q701s making them more enjoyable (but not as neutral). The Q701 does have better bass extension but the AD900X's bass leaves me satisfied.
Ultimately, I paid $180 for the AD900X, $250 for the Q701, and $100 for an amp. While amped the mids, highs, and soundstage were slightly better on the AD900X and the bass slightly better on the Q701. I couldn't decide which to keep. I didn't think the amp was worth the $100 for the AD900X, so I sold it and am selling the Q701s. $350 for the Q701s and amp or $280 for the AD900X and amp, only sound like, I'd say, 10% better than the $180 AD900X unamped.
