You may also be suffering from acoustic feedback, i.e., the sub is causing the turntable to vibrate, which in turn is picked up by the cartridge, amplified, and played back through the sub for even more vibration. You can test this by pressing a finger on your turntable plinth during play and observing whether this changes or stops the sub's oscillations. If so, tweaking your turntable's suspension may provide a solution.
