Turntable feedback

I was listening to a record on my Rega planar 3 which sits on a 1 inch thick slab of granite which sits on 4 metal vibration isolation points which sits on a tabletop made of a slab of solid hickory that is 1 1/4” thick. Yet through all that I had bass feedback. The feedback went away when I turned the volume down a bit. It was not that loud. I’m thinking the bass energy went right to the turntable and not up from below. Any thoughts on getting to the root of the problem? Subwoofer is a Goldenear which is powerful but I don’t like bass to be unnatural sounding so it is not booming at all.  I like it for its detail not punch. 

The turntable is in a corner nook so I wonder if there is a standing wave in the nook that could be transferring directly to the platter. Has anyone else had this problem?

Thanks