This applies to the Scarlett 2i4 and Saffire 6 USB

If you want to use the two pairs of RCA outputs simultaneously, for example to plug into a DJ Mixer, you need to set the monitor gain knob to its maximum to match the output levels.

This is because the monitor output level only controls outputs 1 and 2 on the rear of the unit, outputs 3 and 4 are permanently fixed at maximum level.