Hi David,

I contacted PG and this was their reply.

That doesn't appear to be possible, but you can do something similar using subgroups on the mixer.

First, you'll have to go to Options - Preferences - Audio, and check the box marked "Unassigned Output Ports Work As Subgroups". Then you can assign each audio track to a specific subgroup, by right-clicking on them, going to Port, and selecting something out of the list. You can then control the volumes of the subgroups by using the volume sliders marked A1, G2, G3, G4, etc.. on the right side of the mixer.

You could assign all the backing tracks to A1 and assign the lead to G2, then use the the A1 and G2 sliders to balance them.