A long time ago, I rewired my Gibson "The Paul" with a series/parallel switch, phase switch and single/double-coil switch for EACH pickup (that's right, I drilled six holes in the guitar to do this - I would never do this today). A humbucker's two coils are normally wired in series. In parallel, they sound very much like a single coil pickup. On my guitar, the parallel sound is very close to the single-coil sound, only there is less induced hum. The normal series wiring produces by far the best tone, to my ears.