Interesting problem.
Since the bass notes will draw the most current I would suspect either
a bad filter cap or the power supply isnt regulating ( if a solid state set).
If its a tube set a few were known for bad vert out transformers. They
gave very distorted vert & there would be wax all over the place.
I will pull up the manual tomorrow if I got it.
73 Zeno
LFOD !