I had the same problem but to me setting lfo amp in the middle, (till you feel it click softly) took the humm away, i think that every analog synth is different. theyre very difficult machines to take control of because they have al that electricity running trough the components and as we all know electricity is never constant so the analogs arent either. i find the keys very nice to play but the machine itself is not a easy dame to me, sometimes a sound changes spontaniously (much grittier with no reason) just as i set the right tone but hey thats how this machines work, in the old days it was just the same, analogs are not very stable and they do have a attitude but thats a charm of this machines, the longer you have it the more the circuits are going to function better, its like driving a new car, the first 500 are difficult, after that pure heaven.
already looking out for that ms-20 to complement the MB.