The Crown MA-series does have a built in 7Hz filter. So you can discount infrasonics from messing with the channel blowing. I did however have the same thing happen to me with a macro-tech many times, it turned out that one of the comparators was not functioning correctly and did not allow the amp channel to operate.
The channel was not blown (burned junctions in the output devices) but rather that the channel never activated, leading me to think that it was blown.
It was finally fixed by Crown and returned, but your problem might just be that the amp is "messed up" and cannot be repaired--I've known a few cars like that...hmmmm. I would try another one in its slot and see if the problem repeated or if there were any strange side effects.
T