Need ideas about mandolin and loop station sound?

[Jerome]

OK this is an old thread but here goes anyway. I don't know what your setup is like but it sounds like you are trying to amplify a signal that is already amplified. Turn the output of the loop station down and use the amp to control volume. Check for impedance mismatches as well. Feedback might also be causing a problem - i.e. you might be too close to the amp. Does your mandolin have an output jack with it's own internal pickup or are you using a mic to get the output of the mandolin into the loopstation? Finally, make sure all your gear is grounded properly.
The fact that it sounds fine on headphones but lousy on the amp suggests that the amp is picking up a signal it shouldn't be. Normally some type of feedback. The other thing you can try is a DI box in the signal chain.