Powered back up, while still in the service cradle. Now, lifting up the preamp end of the chassis an inch or so, letting it drop back down just generates the immediate transient, while not exciting anything else, the way it was doing. Pounded on the chassis, tapped on the tubes aggressively, all normal. Put it back into the cabinet, and powered it up, pounded on the cabinet, and now, finally, rock solid.
The frightening thought is how many of the 65 reissues we have of that particular manufacturing date.