The available power depends on the headphone impedance and the signal voltage applied to the headphone amp.

OTL amps (Crack, Crack-a-two-a) produce the most power into high-impedance phones; amps with output transformers (S.E.X., Mainline) produce the most power into low-impedance phones.

For the specific question of Crack-a-two-a vs. Mainline, Crack-a-two-a makes more power into impedances greater than 150 ohms, Mainline makes more power into impedances less than 150 ohms.

Some phones or tablets have low output voltage and need additional gain to get the maximum power.