don't understand monitor function in cuemix fx

[verschel]

Hi,
I'm new using cuemix fx with a 896 mk3.

example"
input analog 1: mic

assign it to a mix bus(1) routing analog out 3-4

analog out harware connected wit a headphone amplifier, so I can hear mic 1 there

now I want listening, to control the mix on the phones from the Motu (assigned to main out)

So in the outputs tab I enable the monitor button for analog 3-4, the label for the monitor output changes from main out to custom, in the outputlevel meter I see that the signal is incomming.

But I can hear the mix assigned to bus 1.
So I can't control what the singer in the studio get's in his headphone.

Can anyone help me?
Thanks