I would start at 2 ohms. 100dB/1mW is quite sensitive! On the 2 ohm tap, I think you'd have around 80mW into 100 ohms, so way more than enough power to say the least.
For connecting a headphone jack, would I just solder the grounds from both right and left and connect it to the ground on the headphone jack, and the right and left to their corresponding places?