I am guessing the warranty has lapsed now? Shame as that would be something that the warranty would have sorted via a repair or replacement.
Do you have any other amps in the house that you can try the pre-out theory with? It is probably worth checking that the crackling isn’t there before buying an amp.
You would run two RCA cables from the Surround left and surround right pre-outs into a stereo analog input of an amp. Then connect the surround speakers to that second amp. Switch everything on and play the test tones, then increase the volume on the second amp to get the level of the surround speakers in the right general region. Then run Audyssey again to fine tune.
This is s good low cost power amp that would do the job:
Studio Power Amp Linear power amplifier for studios, events and Hi-Fi applications, Convection-cooled for silent operation (no fan), Protection circuits for DC resistance and overheating per channel, Fast-response switching power supply with high…