Hi, I kind know what your mean, you want adjust sub-bus seperately from guitar-bus, then both can be adjust in Cue mix. right?
in DAW, Cue seems only avaliable in Track\Bus\FX, but not in sub-bus, I dont know the theory, but its still a way to approach it,
when your signal transport to next bus, you can bulid a pre-fader send to that bus too, until it finally arrive to Cue mix. So you dont have to create seperate Bus or FX, just figure out the signal workaround.
you see, the guitar track assign to sub-bus, then to guitar-bus, then to main. but the prefader send do the same thing one way down to cue, lot of freedom to control your sub-bus value in guitar-bus.,