A popping 4650 has arrived here yesterday and will share my experience with the above suggestions.
I tested the unit out of the box and was unable to replicate the popping speakers problem. Powered on and off a dozen times when the amp was cold and after half an hour warmup, all inputs... no plop, crack or anything. Nevertheless I cleaned the power switch, soldered a 1uF 280V spark suppressor on the switch and replaced the protection relay. Shipped the amp back to the owner this morning and awaiting his findings..
Update: Happy owner reported that the popping is gone. Will add this mod to the list of modifications, expecting another one soon for restoration. Thank you!