I have a Martin 000c-16rgte Aura that I bought solely for gigging. Plays real easy, and I can do leads on it, etc. (I leave my Taylor 810 at home.) However, even with the rubber sound hole cover, the guitar feedbacks very easily. I switch the phase, I try to identify the note, etc. Nothing. It ALWAYS wants to feedback when I play the open G chord. Is there anything I can do? Stuff a towel inside?