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The turntable is in a cabinet about a foot off the floor
and right next to the left channel speaker.

I'm reminded of the old joke where a patient tells his doctor that his arm
hurts when he holds it vertically. "Then don't do that!"

I remember when H H Scott made console phonographs. They went through quite
some trouble preventing feedback when the turntable was in the same cabinet as
the speakers. And I doubt their speakers went as low as the JBLs.

Some of the suggestions are good (such putting a huge chunk o' granite under
the 'table), but you really need to move the turntable away from the
speakers -- and the walls.