The speaker swap was to try one of the fronts as the center to see if it still made the crackle sound.
I wish I could return the speakers and try something else, but that wouldn't be an option. If it's a problem with the speakers, the best I can hope for is that it'll be fixed under warranty.
Next up is to try the PSBs as the fronts/center and see if it still makes the sound.
If it still does, then I'll try a new AVR.
If that doesn't help, I don't know what else to do.
I'm kind of doubting that it's the AVR now, especially since I never had this problem with the PSBs in the front.