On the MPC X and X SE, there are multiple output ports, for connections to monitors, external mixers etc. It is important to note, that the additional output ports (channels 3-8) send out at full volume (0dB), regardless of the position of the main volume knob.
This means that if the main volume is at a lower level, the main outputs will be quieter than the additional outputs .To adjust this, you can change the position of the main volume knob to match the output level of channels 3-8.