Burned voice coils means too much power. This sounds like the case of improper gain/volume settings, or an improperly sized speaker/amp combo. You say you had the gain "turned down"... there really is no turned down, there is a proper gain setting, and then there is everything else (for the average user). When someone says they had the gain 'turned down' or 'set low', these are signs that they are using the gain knob as a volume knob, not as a set-and-forget adjustment.
I dont see any discussion in this thread about the actual sub and amp used, or the enclosure size, either.