Sven450,
I too have the Roland Octa-Capture.
I connect My Presonus HP4 headphone amp to the 3/4 out of the Octa Capture.
In CbB, I uses sends to send a copy of all the Music and the Vocalist to a Cue Mix Bus
The Cue Mix Bus is routed to Out 3/4 of the Octacapture
I can then adjust each send level for each of the tracks based on what the vocalist needs to hear more of or less of.
Remember to use Pre Fader sends for the Cue Mix so that the singer can still hear the entire mix in case you want to mute or solo a channel in the control room.
The HP4 is near the singer so they can adjust their own levels and I have everything routed through the original Mackie Big Knob which provides me with 3 separate monitor outputs, 4 separate audio inputs, Mute, Mono, Dim and the ability to speak to the Singer while they are listening to or recording vocals.