Nothing wrong with the HD-598 without an amp, although it's not suggested.
I use them all the time with a portable CD player and a tiny Sansa Clip and they're still quite good.
You don't need an expensive amp for them and even a portable amp would be fine.
IMO a long as they're driven well enough, I haven't found that they benefit much from amp upgrades.
Not like the HD-650 or even HD-600.