Sounds like a fried tweeter to me. I think what causes it is when the leads break, the strands tend to be touching at low volume but at high excursions they lose contact and then regain it, causing a crackling sound. I recently had this problem with a Thiel Viewpoint I got off of ebay. It was damaged in shipping and I glued the woofer together and got it working. The tweeter was crackling though, I took it apart and saw that strands of the wire leading to the voice coil were broken. I ordered a new tweeter and it had the same problem almost immediately. I took it apart again and the leads were broken just like the other one. I measured the crossover output for the tweeter at 60hz and it was the same as the output for the woofer. The crossover was damaged in such a way that it was frying tweeters.