Having GTR , vocals and EHX B9 all mixed by the QC means that the main volume affects all these instruments.
I found a way to solve that.
If I remove the out on the vocal line ( to X not used) and then put an effects send at the end of the chain( but not at the outputs) I end up with a line out that is I unaffected by the main volume out.
I route this out into a harmoniser which then puts the vocal mix into a stereo mixer channel.
I try keep the vocal level much the same throughout the gig and although I have the GTR etc levels pretty well set I can just alter the QC volume to fine tune the level for different songs. Different pickups and guitars have slightly varying outputs.
Love this machine.