Hmm could you clarify this to me. Because after looking at HD598 specs they are pretty easy to drive. You only need around 13mW to reach 110dBSPL with these headphones. I don't see how having something big and strong would make a difference to driving these headphones when they need so litte power.
Getting headphones loud doesn't mean that they're powered properly. The need a lot of power and current to work well. You can get them loud out of an ipod, but they won't sound nearly as good as out of an amp. Loudness is not sound quality. Also, their impedance is 50 Ohms only on paper, its actually not constant, it has a big spike that goes up to 300Ohms. You can use them without an amp, but you'll be wasting your money. You better buy cheaper headphones then, you'll get the same sound out of them.