Did an outdoor gig yesterday at a big ampitheatre. The soundman ran a line out of the DI of my GK combo. The amp started breaking up, especially when I "dug in" on the bass. It was pretty noisy coming out of the amp, as well as the line that the house had. Funny thing, though, when he took the XLR out of the amp, the breakup disappeared(?). My sound was clean as can be, so I went into a direct box instead. What gives? Anybody ever have this problem with a GK MB amp before?