I've already done this. The issue is not with the side fills.
But you have not individually tested the four 5" speakers and 12 dome tweeters in the little array.
Since you run the speakers at or above their maximum rating, any one of the 16 drivers could have a voice coil dragging, and only be "excited" by a combination of frequency and level that a certain vocalist achieves.
As has been pointed out, the best way to determine if a driver is bad is with a very slow sine wave tone sweep, pink noise or stepped tones may not activate the distortion.
When running speakers over their maximum rating (300 instead of 200 watts) and at around the maximum SPL they are rated for one can expect distortion and meltdowns, if you are lucky, the speakers have not been damaged.