Whoa. I don't think you realized you just posed a deep question.
The situation of picking an amp for certain headphones is complicated and involves multiple issues. There's an issue of source (DAP? CDP? et al), scaling (how high up the chain should you go?), features (since feature sets vary wildly in the amp market), aesthetics (can be important for some people), physical and technical specs (weight & dimensions, power output, etc), and so on. And sound, of course. You have to take into account all those things when picking out an amp from the crowd. Of course theoretically you could pick any amp at random, since nearly all dynamic amps will drive nearly all dynamic headphones, it's just that varying degrees of success and synergy would result.
You pretty much just have to decide for yourself what YOU want in an amp and then look for the amp that fills those criteria, taking into account the source you'll be running it from, along with your headphones. Nothing is cut and dry, and then there's lots of personal preference involved too. And notice I'm not even talking about sound yet.
I've noticed a trend lately of people around here arbitrarily and spontaneously picking amps without really researching what they're getting into, and instead getting something just because everyone else is getting it, or because it's being advertised, or because of some other factor. The thing is, each and every amp is unique, and it's up to you to decide what's important to you instead of pursuing something you're going to decide later will be useless and not what you wanted. So many Head-Fiers follow compulsion way too easily (just track the FS forums for a week and you'll see what I mean) and end up blowing through money to get to something they discover they didn't really want after all.
The best advice I can give after my own trials and errors is to
research and
audition wherever possible. If there's a meet in your area, do what you can to attend. And then keep reading and learning. Don't buy something just because it's popular or just because it gets a great review from one publication. Find out as much as possible.
Then, when you think you have all the data you need, research
prices and determine if you want to buy new or used, and then go for it.