I used to have the older AT-933/c's (CMC-8s) from the early 2000's and they didn't have the 4.7k mod on them and I would get this if I was "too close" to a loud source. It sounds like it's either overloaded at the source (the stacks) or the mics were overloading.
I would also suggest you *never* stand dead centre within 15-20 meters from the stage as you end up getting a "bass-doubling effect and miss a lot of the sound around you because the stacks are too far to the sides. I'd suggest being in line with one of the stacks for better results (or since you're running cards), stand back about 30-40 meters from the stage.