Hi
Tone2 Pro supports both 4.4mm headphone and 3.5mm headphone.The connectors are showing below.
If your headphone is 3.5mm,you can plug connector ① directly,and if it is 4.4mm,you can plug ②.
And then you should chooes right gain mode before you start to enjoy your songs,you should know the impedance of your headphone.You can find the answers in this post.