Althought it doesn't exactly match the background of the OP's problem, I would like to add some experience I made with exactly the same as the reported symptoms on a Marantz 4270. My reason for adding this here is that both receivers are quite similar and when I searched for a solution regarding the "loud popping noise" on a Marantz where even the protection relays switch off ("sound disappears for some seconds"), I stumbled over this thread because it comes closest to at least the symptoms I had on the 4270.
Description: So, yes, the initial problem was that after switching it on, the receiver began to produce heavy pop noises over the speakers on both channels after just a few minutes. Speaker protection switched the relays off for a few seconds. The pop noise came back a few minutes later, but went quieter the warmer the receiver was. It eventually disappeared for the time the machine remained switched on. The noise increased with »bass« turned up (and vice versa) and became very quiet with pre and main amps seperated.
Diagnostics and therapy attempts: I found defective caps in the power supply stage (especially the glued ones, but I replaced them all), replaced all the caps in the sound control stage, replaced one relay, also the eight differential input pairs of the main amp (they actually had significantly different amplification factors; I replaced them with 2SA1015), but the problem remained.
Finally and accidentally, when touching the power supply board, I realized that "jiggling" the cooling plate with its two 2SD331 a bit back and forth immediately triggered the pop noise activating the speaker protection. I checked for cold solder joints but could not find any. Yet after resoldering the whole board, the noise finally disappeard completely.
Having found the cause, it is easy to tell why this must be a (more or less) common problem. The cooling plate heats up and cools down, and the way the power supply board is constructed, it inevitably bends the solderings and junctions of and around the voltage regulators. Some recommend to restore this board in any case, and I think that this far from being exaggerated. Besides, it remains a very good receiver, most other caps still had at least their initial values.
Hopefully this might help someone else in the future.
Cheers
Thomas