Heeding your caution about costs, I decided to do some more investigating since the chassis was already out of the cabinet. Before I even started probing for loose connections/wire I noticed something else, a blob of what I assume to be white thermal conductive paste on the edge of the PCB adjacent to the leads of the power amp chip (right side as viewed from the front). This gunk was on top of some of the SMT pads where some of the caps dropped from later revs of the amp would have been. I assumed that if this stuff is thermally conductive it might be electrically conductive, or at a minimum, have some sort of resistive or capacitive effect. At any rate, I cleaned this stuff off the pads, hooked things up and, voila, the amp is working like a champ. I'm still wary since it used to fail intermittently, but I'm cautiously optimistic that this was the source of my problems.