I replaced the D-A converter, but the  0.8 MHz sinusoidal output ripple remained.  More surveying, with the D-A converter and a few other ICs pulled, demonstrated that ripple was present across the groundline itself, observable by probing the ground line at the rectifier board and the ground point in the vicinity of the power transistors. Lacking a good reference point on the main board threw me off when I was investigating the D-A converter and saw ripple at every Analog point (should have been obvious the problem was more fundamental).

So, let's consider some vintage equipment repair fundamentals 1) look for burned resistors and transistors (done) 2) check your electrolytic caps (not done, oops). Checking electrolytic caps includes checking ESR (or just replacing key electrolytics with modern low ESR parts if they are cheap enough).

There's only one electrolytic per channel on the main board, at the output of the power/voltage gain stage. I pulled the output capacitor and noted that CH1 was 100 uf while CH2 is 200 uf. (There are mechanical reasons for the difference, but there are also workarounds, so there's an opportunity for improvement).  replacing with a decent low ESR cap solved the problem of ripple on the ground line entirely. But there's now intermittent ripple at the output and at the negative 15 V rail.

With the ripple reduced in magnitude and intermittent I've re-probed the main board but can't find any signal matching timing or phase with the intermittent blips of ripple.