I understand that. What I mean by the question is that if I have a headphone how would I know when buying a dac/amp it will be a good pairing? I can figure out how many ohms an amp will power but isn't there more to amplifiers than that?
You want to know where a headphone is in terms of its signature [the way it sounds]. A headphone like the X2, you want to keep the output impedance under 4 ohms [I believe the impedance shoud not exceed 1/th of the headphone' impedance], though higher impedances might be chosen if one wants to color the headphone in a certain way.
So knowing what the headphone sounds like, X2 being very good bass [both extension and decent impact], decently present mids and slightly smoothed treble, you are looking at a solid state neutral to slightly bright amplifier, depending on budget there are choices.
With the X2, matching is not a big issue, but you don't want to be using a warm sounding amplifier or something tubey.
It is more a matter of pairing and synergy. You usually look for an amplifier where you want to change something on a headphone or feel like it is missing something, then you go out and look for an amplifier that suits that desire. If you like a headphone as is, then that's obviously also great