On the EVO 8, connecting a set of headphones to the headphone port will automatically mute your monitors for that channel making switching between loudspeaker monitoring and headphone monitoring quick and seamless.
For example, plugging a set of headphones into headphone output 1 will automatically mute the speaker outputs 1+2 and plugging a set of headphones into headphone output 2 will automatically mute speaker outputs 3+4.
Therefore, if you’d like to have monitors and headphones connected at the same time, you’d simply need to connect the speakers to Outputs 1+2 and the Headphones to headphone Output 2 on the front of the unit.
This allows you to set different volumes for the two outputs and using the EVO mixer application, have different mixes playing out of the speakers and headphones. More details on how to do this can be found here.