Wildly fluttering woofer

It sounds like subsonic feedback. Move the speakers to see if it improves. Also try to isolate the turntable.

You can easily isolate the problem by swapping channels from the speakers back to the turntable.

It is almost certainly sub sonic feedback as mentioned before. I have seen this using test records on my own gear which can't produce a sound below 28 cycles but will wobble violently between 20 and 26 cycles.

Had same situation. It was my phono stage. Got it fixed, problem solved

Thanks for your suggestions. I've recently movedy speakers and placed components on a new rack. Soon after there was some stylus instability/wobbling. I placed three products from Herbie's Audio Lab and cured the problem. I sent my Audible Illusions pre amp back to Art to check it out. Anticipating a wait, I found a Yamaha C60 preamp,used of course, to tide me over. I timidly tried some vinyl and no flutter. ALSO, my cartridge was tracking much to heavy. I'm hoping Art finds a problem with the preamp that he can fix and with the other steps I've taken I can listen without fear of a woofer fatality.

So the problem was not feedback.

A noisy tube in the phono section is still at the top of the list.