I notice the the out from headphone (actualy I cannot try the master out) is good olny if master volume is to max. if you low it the sound stereo --> mono. is simple to try. play style and mute all tracks and play only a guitar that stya major in lef or right. more I close the master volume, and more the out go from stereo to mono.. this append on headphones, and ear only if mute bass and drums and low the master cursor. that if stay at max no problem.

and the other 4 out ? is not possible to replicate the master ? for example use to personal monitor ?