What comes to mind is it might be a ground problem between your arm/cartrdige and phono pre. The fluttering may caused by infrasonic frequencies causing your woofers to flutter. In my case, Dan Wright came to the rescue and sent me some resistors, and told me where to install them, and it worked like a charm. There are filters that can be had that will filter out those frequencies.
Back to the hum problem, it's also possible you have a ground loop problem. I have had one in my center channel for a couple of years now. It only happens when the prepro is turned on. I've tried everything, an isolation tranny on the speaker wires to the center (that helped a bit), isolation on the power to the center channel amp and prepro, it just won't go away for me. I hope your problem will be easier to solve than mine.
Good luck, regards,
Dan
Back to the hum problem, it's also possible you have a ground loop problem. I have had one in my center channel for a couple of years now. It only happens when the prepro is turned on. I've tried everything, an isolation tranny on the speaker wires to the center (that helped a bit), isolation on the power to the center channel amp and prepro, it just won't go away for me. I hope your problem will be easier to solve than mine.
Good luck, regards,
Dan