#1
What exactly is a "stacked humbucker" ?

Carvin's custom basses have the option to switch the stock j-bass pickups for "stacked humbuckers," and on their site it shows what looks like an EMG single coil.
#2
Hey. A stacked humbucker is essentially the same as a normal humbucker. In a stacked bucker the magnets are placed ontop of each other instead of side by side as normal. This makes it possible to replace noisy single coil pickups with humbuckers without having to make the hole any bigger.