I haven't heard the HD598, but I prefer the HD600 to any closed headphone short of the Ultrasone Edition 8 (SRH840, SRH940, D5000, D7000, DT770, K271, DT250), but those cost ~3x as much. Open headphones almost always sound better than closed headphones in the same price range imo.
He meant the 840s which have the mid-bass hump. A portable amp would not help much. 940s do not have this emphasis, it is one of the biggest differences in their sound.