Not really the most ideal solution, but if you are on a budget I'm using this Behringer amp that I got from Musicians Friend for like 30 bucks. It's pushing my XB700s rather easily.
If you want to spend a little more cash you have 2 options. You can get a FiO that gives you a very nice USB DAC and headphone amp in a nice portable package or you can get a Schiit headphone amp. There are obviously other options, but these are the ones I have personally looked into for purchase.