First of all don't hesitate to call Axiom. But, I question if you have the sub calibrated to the same SPL volume as the other speakers. Did you run a calibration test tone either within the receiver, or some external track to make sure your dB levels are all the same?
My room is very large and my old EP500 and EP600 gain(volume) knobs have to be very low to achieve the same match with my other speakers.
Even with all of that, it is not normal to here crackling even if you were running the sub to hot (to high).
