yes, it's double for two. (it is a pretty massive subwoofer)
i wouldn't know about the hum. i would guess that it's not a design problem, but an amp factory defect which means only a few people would be lucky enough to get the "defective" amps.
how does AV123 handle the people with the hums? do they offer to replace the amp? that should be your gauge if the hums matter or not. if they DO replace the humming amps, don't worry, they'll take care of you.