I have a 4 channel amp run from an electronic crossover and recently the rear speakers have been acting up. Either one or the other will work, usually not both. If I turn up the volume pretty high, the one that isn't working will sputter and come on for a short time, then it usually goes back out. Sometimes the right rear will make a popping sound. The 2 front components work fine all the time. Anyone have any idea what is going on?