Stacked humbucker talk

What is a stacked humbucker?
It's a humbucker that has a second coil of wires placed underneath the other. This is different from a regular humbucker where 2 single coils are placed next to each other.

So is it a single coil or humbucker?

Let's get this right- a humbucker, by definition, is a pickup arrangement featuring 2 single coil units wired to eliminate hum (or 'buck' the hum). Regardless of how the coils are placed, the objective is the same- to eliminate hum so don't let anyone tell you different. When a 'humbucker' is mentioned, there's always more than 1 coiled unit involved. 

What's this about stacked humbucker not sounding as authentic as a true single coil?

Simple- it involves more than 1 coil so essentially, it sounds different from a true single coil. The degree of authenticity depends on the players' ears, really. If you can accept what the manufacturer has to offer, chances are, you are agreeable with the tone in relation to the true single coil tone. The stacked humbucker method differs across manufacturers' designs, so tone differs as well. Some involve a dummy second coil just to eliminate hum while others do not magnetize their separation layers. This happens because each stacked formula is a patented affair so the rival manufacturers have to come up with their own way of doing things.

I'm about to have one installed in my guitar, it should fit, yes?

Please refer to the pic above, the stacked humbucker (black) is taller than the regular (white) single coil. Your guitar's pickup cavity should provide for this allowance. My experience with such pickups is this- the chances of fitting one is higher if your guitar features a pickguard rather than a direct mount. However, I get positive results with both guitar types (Strats featuring pickguard mounted pickups & Ibanez models with no pickguards). Let's put it this way; why would a certain manufacturer be dumb enough to not make their products compatible with many guitar models? They know people are gonna run to their competitors for something that works.