Hi PhilippG,
You would need to connect the H90 to an external mixer or headphone amp in order to be able to monitor the output. If you play guitar and you want to simulate playing through an amp, you’ll need to connect the H90 to an amp/cab sim to get the most natural sound. Some options on the market already have a headphone output built in and will allow you to connect in mono or stereo. Something like the Line 6 HX Stomp comes to mind.
H90 -> Amp/Cab sim -> Headphone amp
