That seems to only leave the AVR itself I guess. A bit of googling suggests that the front right channel may be knackered. I tried resetting all the speaker settings and set up the surround again but it's still doing it.
What can I do from here? Is it likely the AVR channel is actually poked? If so, is there anything I can do about it myself? Anything else left to rule out?
I recently changed my front speakers to bigger more powerful floor standers but the crackling didn't start straight away. Haven't been thrashing them either, just started doing it a couple weeks ago.