In your communications with the company, did they agree to sell you the module and then not follow up with any further communication? I am a bit confused as to how the problem was diagnosed and why the company would agree to send you the parts to do your own fixing. That is quite unusual. Most companies don't want the customer so much as opening the equipment case and have warnings about voiding any warranty if the case is opened.
I am also a bit confused about your switching the two modules to see if that would effect a cure. Do the two modules have identical parts numbers? Did the amp only fail in one channel such that a swap of modules in separate channels could confirm that the failed module was the cause? Did the company recommend this as a diagnostic step? Did you confirm that it was safe to do the swap? If you did this on your own, did you practice proper grounding of yourself to prevent electrostatic discharge damage? It is not clear the extent to which the company endorsed your plan to replace the module and then failed to follow through.