If it's any consolation, these took a while to grow on me, but now I wouldn't ever live without them. I kinda felt the same way too, coming from much livelier Grado cans, and on direct comparison, they may very well seem laid back, but whether it was just brain burn-in, driver burn-in or both, I have grown to love my HD598s for their unique character.
They have a certain warmth and liquid like presentation and won't blow your socks off with speed, but that totally non-fatiguing and smooth nature is just what I need sometimes for late night listening and these get more head time than my HE-400s.
They are supremely comfortable as well, which is something I don't get with my other cans.
BTW, I listen to a vast range of music, but mostly large scale orchestral stuff, with a lot of metal, jazz, funk, big band stuff in there too.
I have a collection of headphones to fulfil my listening needs, but wouldn't be without the HD598s.
Give them a try with a good dynamic movie too, I think you might be surprised just how capable these cans are in their scale and presentation. Also, you hardly notice you're wearing them and they look gorgeous to boot!
I really hope they grow on you before you get rid.