You need a good amp to go with the Sennheiser 598. The Fiio E6 is an amazing cheap and lightweight amp that does the job. It even has a bass amplifier.

The thing is charged via usb. And the battery life is about 10 hours. One of the best things about it is highly portable. So you can bring it on the airplane so that the max volume doesn’t kill you. Whenever I take airplanes… I’m watching a movie or listening to music.. and then the announcement goes to max volume and freaks me out. So… with an amp you can actually lower the input volume to something that you can listen to. The other good part of this is that there is a bass boost. I think there are some headphones that I own that aren’t that bassy… and this amp helps out with that aspect.

The amp actually makes a noticable difference in powering the Sennheiser 598s. It makes the music sound much cleaner as opposed to it being slightly muddy / blurry. It’s so good for so cheap. You should just get it.

tl;dr - get this amazing cheap amp here