Like you said, the omnidirectional field of pickup likely is the culprit. Since it is marketed(according to a website)for educators, dance instructors etc, it is best used far from the speakers, like in the middle of the floor. My Sennheiser is a supercartioid and is very resistent to feedback, unless my gain is set to high.
pa4X 76 ,SX900, Audya 76,Yamaha S970 , vArranger, Hammond SK1, Ketron SD40, Centerpoint Space Station, Bose compact