Drives in front of the stack cannot raise rig volume, i.e. for a lead boost, regardless of clean/dist sens settings.
This is counter to how boost pedals behave in front of amps.
There is some suggestion from Christoph that the volume compensation used to tame level changes from rig to rig is also affecting pre-stack drives. (link below)
If this could be modified, it would allow pre-stack drives to be used as something other than flavors of gain.
It seems an awful shame to waste morph function as a lead volume boost when a drive/boost in front should be able to do the trick.
Boost in X slot is also a poor solution.