Send the signal (song) from the mixer to the RMX and then return (song with effect) back to the mixer. Turn on the XPQ and then then assign the effect to a channel with the selector knob. Turn up the source XPQ volume and you should be good! If not check if Behringer has a forum that may address the S/R on the 750 better. but the RMX is really straight forward with how it works for Send and Return! Good luck!