Recording with Quad-capture - how to mute monitors

[iidolevy]

Hi,
A very trivial one.
I just got a QUAD-CAPTURE for studio recording.
For voice recording (condenser) I want the singer to hear the music only through his headphones and not through main monitors. Problem is, there is no way to mute only the monitors - there is only a one volume knob controlling both.
Is there a solution for that? (it is a very basic functionality for any recording, so I guess I don't understand something).
Thanks