I suggest you to try this:
MAIN out @ 0;
All TrkLev at 127;
Mix the tracks using only VOL in AMP page;
Mix them to a nice sounding mix;
Copy the Part and paste it to another slot;
Select the new Part;
Mix the tracks to a quite distorting mix;
Now raise the MAIN out to +63
Does it distort more or just raise the volume?
Try this MAIN out with the correct mix Part too, for comparison.