The problem can be caused by many things, but input differential transistors going out of mismatch is the most common, followed by fading resistors in the feedback and/or input impedance. Also a problem is an electrolytic capacitor in the feedback divider leaking or having a high ESR.
You're a good tech @gs5556 ; that's pretty much what was replaced in my Nakamichi PA-7A MKII when it was making a popping noise when turned on.