Repedative failure modes are the bread and butter of electronic servicing. Same as a working mechanic. He could not make a living if he had to seriously troubleshoot everything that came in through the door. If he services say mazdas primarily he is aware of what has probably failed by the nature of the complaint and he has changed fifty times earier on other mazda cars. This is pretty normal in my opinion. If someone took a rebuilt amp apart and listed the parts that were obviously changed most likely the same parts in your defective amp could be changed out by an electronic friend or yourself with some experience at very liitle cost. Probably in say 90% of the cases you would be back in buisiness with an updated amp as well. Just some thoughts.