So, despite it not showing any leakage or damage, I replaced C155:
Which had stopped being a capacitor some time ago:
I also replaced C165 with a newer capacitor.
Again, having the hood up, I pulled C452 and C462 from the ±19.3 V supplies:
Despite the 1987 date code, they seemed to be in fine shape, but I replaced them anyway. The new caps have a 50 V rating, not the original 63 V: only a factor of two headroom.
The four new capacitors in their new home:
The power supply voltages looked clean before and look clean now.
The AM503 still has the mysterious 4 MHz oscillation, so the capacitors weren’t the problem. Even though the amp is still sick, I feel better.