Reply to Low main output on the S4

Hi! I'm having some trouble with the main output on my S4, when setting the software knob to 0.0dB and turning up the hardware knob to full the main output level meters on top is only 2/3 or sometimes halfway up, even if i have a track playing with occasional clipping (e.g it hits red sometimes on the channel meters). So, even if the channel is clipping balls and the main output meter in the software is doing so the hardware is only halfway there. Any ideas?