Sorry I missed you stating they are actually crackling.
In my case the input resistors had cooked which also cooked the surrounding connections. All of which increased the input Z causing reduced output. But if this were the case you would likely detect a burnt smell. So that’s doubtful.
Perhaps disconnect the amp and check for output voltages. Both a/c or dc. If you detect over 30mvdc or any a/c at idle send it to be serviced. Stats don’t like much dc offset. In fact as close to zero is always perfered.
If it passes that then ring out you speaker wiring to make sure they are faultless. Perhaps the amp is having issues with the automatic transistor biasing circuits. And is dialing down into limp mode.