That is incorrect. The HD598 has a very different design than that of the HD600 and is much more efficient. So, any quality portable amp would drive it quite well.aren't the HD598's very similar to the HD600, just more plastic parts?
If so, *do* not get an Fiio E11. I just bought one to try to power my HD600's and even on high-gain, freshly charged battery, there is nowhere near enough volume/current to drive them to decent levels without clipping/distortion. Big disappointment, I'm returning the amp.