thanks for the help. I still can't hear the Quack, but maybe I just don't know what quality of the sound everyone else is referring to.
It’s an out-of-phase sound caused by the different locations of the middle pickup and whichever other pickup is selected. The phasiness acts like a formant filter for certain frequencies (describing a formant filter is too tedious for me, Google it) giving the notes overtones that remind folks of clucky or quacky mouth sounds.
It’s difficult to describe sound with words, isn’t it? If you can hear that positions 2 and 4 sound different from the other positions, that’s probably enough.
Back to the actual topic for a moment...
Although there is a tiny amount of phase cancellation between the double pickups in a humbucker, I don’t think they’re necessarily less clear than single coil pickups, but the impression of single coil clarity is certainly enhanced by the fact that there’s less lower-midrange in many single coil pickups (remember, P-90s are single coils that have plenty of low mids!).
Pull back the lower mids on any pickup, and you’re going to get less wooliness, a brighter upper midrange, and therefore the impression of greater clarity. This is caused by two things:
First, the amp is less driven in the lower midrange, so it will distort less and operate more efficiently; and second, there’s a psychoacoustic phenomenon known as
masking where certain loud frequencies can predominate over softer frequencies and therefore mask the softer frequencies. In this case, the highs are the softer frequencies in terms of volume. And the thicker lower midrange masks the high frequencies a little bit with a humbucker, especially into a tube amplifier, where the lows tend to predominate as tubes go into saturation anyway.
Last week I posted a comparison between various tones using a couple of EQ pedals that I think effectively demonstrates that cutting back lows and lower mids on a humbucker pickup mimics single coils pretty effectively. All of the segments in the clip were cut with a stock 594 into a clean Mesa Lone Star (with a tiny bit of grit from an OD pedal).
Even merely goosing the high end seems to increase clarity because the masking is reduced. Check it out, and be sure to read the descriptions of what I did:
http://forums.prsguitars.com/threads/eq-or-pickup-swap-a-quick-eq-demo.32372/