You didn't say how you were using the Green Bullet, so I will assume you are using it in the standard manner, cupped in your hands right up against the harp (stupid question, but it never hurts to ask, because I've got my Green Bullet wired for XLR and mounted on a stand).
Any live situation is all about gain before feedback. The solutions to the problem are increasing signal level (playing louder), increasing distance from source to speaker (20' should be plenty, but consider orientation of the amp), decreasing distance from source to mic (probably already all the way there).
Lastly, you can try to isolate the frequency that is causing feedback, and EQ it out with a parametric or 1/3 octave graphic. One method that can be effective is to turn down the master on the amp, mic it, and run the EQ'ed signal through a floor wedge, that way the signal in the mains is not compromised. However, even if all you have to work with is the tone controls on the amp, that could help.
