Generally, I believe headphone impedance is simply the DC resistance of the wire in the headphone driver itself.
For headphones, we seem to see impedances between 16 and 600 Ohms.
For an amp like the Crack, driving 600 Ohm headphones will be much easier than driving 16 Ohm headphones.
For your cell phone, driving 16 Ohm headphones will be much easier than driving 600 Ohm headphones.