I just aquired this unit from a friend who was throwing it away because he thought the speaker was blown. I tested the unit and also thought there was an issue with the speaker, however I changed out the sub with a good working one and the same problem occurred. The problem is when hooked up through the low level RCA inputs, the sub has a crakling sound to it when the bass hits, I have changed between the two speakers and both have the same problem, I have tested the speakers in a different setup and they are both functioning properly. Where do I go from here?