I checked some other forums, and the crackling and distortion issue might something more mundane: there's a single loose strand of copper speaker wire that's touched the chassis or the opposite polarity.
If it were a modern AVR setup, the protection circuitry would immediately shut down and flash a self-diagnosis message to check all wiring. A pre/pro + amp setup wouldn't have those failsafe mechanisms.
It could also be a wiring issue in the center channel itself, but I would check that last. I figure it'll be a 1 or 2 hour job on your hands and knees to re-twist all the speaker cabling ends, maybe more if it's in a closet or hard to access spot. This would be a good time to recruit some child labor.
