Did you try William's suggestion of unplugging the RCA cable from the AVR and tapping the plug end centre pin with your finger? If the sub amp is working you should hear a light buzz when you touch the pin. You can also connect a 1/8" to RCA adapter cable to a mobile phone headphone jack to play music directly on the sub, but since it's a full range signal and the sub has a low pass filter, pick some music which you know has deep bass, like modern electronic music. If those tests fail then the plate amp has likely failed.