Pixies for sure.
since no hypothesis offered so far really holds water, who is to say that a step up transformer would not eliminate the problem? But do you wish to continue to operate in balanced mode? I thought you got rid of the problem simply by converting to single ended mode. If you are happy with single ended mode, why think about a step up transformer.?
Is it really the case that you’ve been using a hi-pass filter operating above 18Hz in all cases? You earlier stated that if you move the platter manually at sub 33 rpm, presumably at and below 18 rpm speeds, the pumping occurs with frequency proportional to platter manipulation. That seems impossible so I must misunderstand.