The HF grayhill switch had exploded itself inside, the tiny contacts had gotten cross-wise with the rotor and then destroyed the plastic rotor. Like a box of pinball parts. I had another multideck Grayhill so I rebuilt it, and realized that the molded portion that carries the pin was twisted, so the knob could only be turned clockwise. Two or the counter clockwise clicks and it bound up again. Never seen that before!
Anyway, with that off I confirmed that the first op amp in the EQ was oscillating when both the high and low filter circuits were switched "off". Added a feedback cap to the op amp which cleaned it up, then I chased my tail for about two hours trying to eliminate phantom pulsation caused by the interaction between my signal generator and the cable going into the scope.