I wanted to follow through on my earlier response. My PG is serial B035037, so similar to Dan's I think. It also shows the same negative going edge distortion you show in your posted picture at 100 Hz, and pretty well cleans up around 250 Hz. Output voltage into 1 Meg is 64V P-P, at maximum amplitude setting, across the frequency range of the instrument.
I too saw the reference to the rising edge being the "reference" edge, so after repairing the PS issue, I just accepted that they (Tek designers) didn't care too much about what was happening to the negative going edge at longer periods. It's good to know that at least two of you are seeing a similar thing. My ignorance is keeping me from understanding why the designers would have chosen to regulate the entire switching supply rather than just locally regulating a fixed -72 rail.
And on the -72 supply diode replacement, I ended up just trying some 1N914 diodes, which seem to have worked well. Also used those to replace CR80 and CR125, which had taken an absolute beating as a result of the strange switching transients that resulted from the "backflow" due to the slow rectifier recovery times. 1N4148 seems like it would work too.