If you've using the balanced (2.5mm) output it looks like max power is 250mw in to 32ohms. Not bad for a portable in that price range, but you'll definitely want to stick with higher efficiency headphones. Beyond that, don't have any specific suggestions. Maybe Phillips Fidellio X2HR? They're pretty easy to drive. Are you going to be listening in loud areas, do you need noise isolation / closed back?