The sonic differences between series, parallel and split sounds also vary between vertically stacked pickups (single coil size) and side by side pickups (traditional humbucker).
On a stacked pickup, series and parallel are both hum-free sounds with series being the louder and fuller of the two. The series sound replicates the sound of a single coil but without the hum. The split sound is a true single coil with all the wonderful tones we expect and the hum that goes with them. This sound is also the loudest of the three sounds. This is because the two coils of the pickup are not hearing the string equally due to their stacked configuration. Parallel is a cleaner, lower-output, hum-canceling single coil tone.
With a side by side humbucking pickup both coils are hearing the string equally. The series sound is the traditional strong, mid-focused humbucker tone. It is louder than both the split and parallel sounds. The split is a true single coil and the parallel produces a weaker humbucker tone.