depending on what type of amp it is (class AB or class D) there are some things you can check. if it's a class AB, it's possible the DC correction capacitor is dried out. this would reduce the gain of the amplifier to 2 instead of 20. if it's a class D amp, it's possible one of the electrolytic coupling caps on the output is dried out. best test for these in both cases is an ESR meter. if you don't have an ESR meter, a similar value electrolytic with meter probes attached will allow you to bridge the suspect cap while the circuit is operating. if the sound level returns to normal that;s the one to replace. make sure you observe proper polarity with electrolytics, as a reversed one is likely to "let the smoke out"