Well, this is the classic example of when a software EQ can only do so much before hitting a brickwall.
You can't have everything in audio unfortunately. Either get a different headphone closest to the type of soung signature you like (in this case, it sounds like an AKG K612) or just deal with it.
You could continue to tweak your EQ further, but I generally believe that software EQ can't do much before completely altering a headphone's flavor into something else, which at that point makes the headphone meaningless.
I have the AT AD700x headphones, which have a great soundstage and already pick up the sounds I want. My setup is strictly for gaming. You're right though, I'm trying to get every ounce of audio I can sweeze out of a software eq to pwn, which I know aren't as good as hardware eqs, and only go so far. If I go external, I lose the surround and other benefits of the SBZ software for gaming, so this is the best option as far as I know.
If anyone else has a better suggestion, I'm all ears!