Boost Headphone Volume?

I got some new over the ear headphones and I have to turn up the media volume up all the way but it isn't loud enough. I don't really want to carry around an external amp at the gym.

I'm not sure of the impedance of the headphones. I've seen that there's a mod on here to increase the volume but does it decrease the audio quality? What options do I have?

I'm on XPower 3.0.