I get the same problem with my Line 6 Vetta II
when I go from clean sounds to heavy ones, even if I have the distorted sounds at half volume. There's a physics theory I can't remember the name of that shows how the highs and lows fall off as you go up in volume. i.e., when we perfect the balance in a solo environment it's fine, when we turn it up with the band and go into a lead, we disappear. I'm working on re-eq'ing the lead sounds with the band now, and it's been working so far. If I can find the name of the theory, I'll post another reply. Good luck!