Pick your headphones in the calculator above and the three fields below fill themselves in. Every source is then judged on three things at once, not just power: whether it can drive your headphones, whether its output impedance is low enough to leave their frequency response alone, and whether its own noise floor will be audible as hiss.
That last pair is why the answer differs so much between a full-size headphone and an IEM. Full-size cans mostly need power. Sensitive IEMs almost never do — for them, a source that is quiet and low-impedance matters far more than a source that is strong, and plenty of powerful sources fail at exactly that.
Enter your headphone specs above to see matched amplifiers.
Every source in this matcher also has its own page, with power into four loads, noise, output impedance and a full headphone list. Browse all 53.