We had bad feedback problems last night playing outdoors on an open-air patio. Feedback was primarily through the mains (stage monitors off). Nothing much in front of us for reflections. We did have a wooden wall behind us and a glass wall to the left of us (we were tucked in a building cutout at the corner of the patio). There was a roof above us extending to approx 8 feet past the mains. The left main speaker (and sub) was very close to the left glass wall. Our mains are 12" yamaha DXR. We also have a 15" sub as base for one of the mains. We route a mono signal to the mains through the sub's crossover. The worst feedback mic was the drummer's vocal mic who was furthest from the mains, in front of the back wall. We aren't a loud band. Our volume allows customers to converse comfortably. So how were we getting feedback from the mains with nothing in front to reflect off of? To be honest, we've been having feedback problems at most of our venues. I just thought that this one, being outdoors, would be feedback-free. Any ideas?