I had a tech mod an old big box silver qtron for me to address that lack of a master volume...only problem was that the way the tech did it......it impacted guitar signal level even when Qtron was off. So it was great wth QTron on...I could crank up the input sensitivity, have my D'Allen single coils trigger the filter and tame the output so I wasn't obnoxiously loud vs my bandmates. But then I had volume cut way down with QTron off.
So be careful of that - be clear with tech what you want with QTron on and off.
I solved the problem by buying a simple SaturnWorks volume pot pedal (see reverb.com) and using that on the Qtron output channel. YMMV
Of course utlimate solution was ditching the QTron (Which has a great sound) and going for MicroTron (pretty good sound, small footprint, master volume)