Ok - there has been a weird issue my last couple of gigs. It seems that a few times during a gig, a particular note (low D) will seem to trigger some very low frequency boomy feedback from my drummers kick mic. When it starts, if I mute everything, it will continue until he zeros the kick mic gain. He has turned the mic gain down, which seemed to help, and I've slid my rig a foot or two farther away from the kick - which also seems to help, but there is still a bit of an issue. I'm starting to wonder if a high pass filter might be the trick - could it be there is a sub frequency that's causing the issue, that the HPF would clean up? Has anyone ever run into this before? Thoughts on solving the problem?