Don't play with the gain. Both channels are affected the same way,
and that pretty much says that the problem is in the vertical output
amplifier. First thing to check is the collector load resistors that
go from the board up to the terminal post. If they're the beige,
rectangular, silicone-covered resistors, they may be your problem.
They're double-counterwound, inductance-cancelling resistors and a
common problem is that one of the two internal parallel windings
opens up, increasing the resistor value by a factor of two. Those
were Tek-made parts and Tek quit making them (they were very
troubleshome) and started using outsourced parts. If one or both of
those resistors have an open winding, they'll be inductive, the gain
will be 'way low and the pulse response will be a HUGE ringing.