I have no firsthand experience here, but I'd give both the drivers and your ears some time to burn in before making any final judgments. Most people here tend to believe in driver burn-in (not sure where I stand on it), so it's quite possible the sound at the high end will open up a bit. You may be able to get a better idea of what to expect from some of the bigger HD 598 threads like this one and this one. Either way, your ears will take some time to adjust to the smoother highs after listening to the brighter AKG headphones. As Gwarmi said, report back in 50 hours. 
An amp may very well help with the male vocals though, and I've read advice suggesting one for the HD 598's: The HD 598's have an impedance of 50 ohms overall. That's a bit borderline, but they have a huge spike in the mid-bass range, where their impedance reaches about 275 ohms. The spike peaks at 100Hz, which is lower than most male vocals, but it extends into a relatively wide range before tapering off completely, and that range definitely includes a lot of male vocals. You can check out a graph of their impedance vs. frequency here:
http://graphs.headphone.com/graphCompare.php?graphType=7&graphID[]=2851
Judging by the graph, I'm not too surprised that male vocals aren't being presented as well as they could be. The E7 should definitely help, but the spike is so big that a more powerful amp would probably still show gains.
You'll probably want a bit more confirmation before going and blowing money (or switching headphones) though, so I'd wait for a more experienced poster to give you a more concrete answer about exactly what the HD 598 require (you can also probably find one buried in existing threads too). In the meantime, this should at least give you a clearer sense of what you're dealing with.
Edited by SobbingWallet - 11/1/11 at 8:27pm