Headphones

Impedence and sensitivity are both important for power requirements for a headphone. While the 6X0 series has high impedance it also has high sensitivity making them not that hard to drive.

There are lots of calculators online that show you the actual amount of power required. I like the headphonesty one because it shows a list of devices that can drive the given headphone (though the list is a bit outdated) and you can link to specific input settings like this.

TL;DR you can literally drive the 6X0 line using an Apple dongle of most (though not all) modern smartphones. Basically any desktop amp unless it is strangely underpowered should run them fine.

As a side note if they currently get loud enough for you on whatever device you're currently using you don't need an amp unless you're trying to remove background noise (say from a PC headphone jack) or add features (like a nice volume knob, more inputs / outputs, etc). An amp won't change the sound in 99% of cases unless you get something specialized like a tube amp or something specifically designed to 'color' the sound.