Any thoughts on why Fender started out using the scheme with the cap wired between the pots then changed to grounding it on the tone pot?
Probably because of the type of caps used. They used to use a larger longer barrel shaped cap, so it was easier to attach that way. They used those caps because they also used them in their amps. Modern caps are smaller, so it's easier to attached them to the pot.
But the basic circuit is the same, one side of the cap goes to ground and the other to hot. In the Duncan diagram the pot comes first, and in the
Stew-Mac the cap comes first.
Also keep in mind that the Fender you know today is not the same company that Leo started. He sold that company to CBS in 1965, and then they sold it to some of the employees in 1985. In 2004 it was reportedly for sale again.
But who ever owns it, manufacturing methods change, parts change, and they find some cheaper ways of doing things, such as the plastic bobbin pickups.
It was not wired up one way because it was better, it was just the way it was done.
Also, vintage P basses are not muddy at all. They sound pretty much like the current ones. What you hear on old recordings was the tone of basses that was popular at the time. They used flat wound stings, and string mutes, and the amps were not as clear as today. Also the older flats did not sound as bright as todays flats. The electric bass was still trying to sound like an upright, because that's what the music called for.
Here's some real isolated Motown tracks. All of them were done with P basses. Hear all the high end in those? Most of them are not muddy at all. It's a myth that the Motown sound, or that P basses are very bassy with no high end. You just don't hear the highs in the mix, but it helps the bass have definition. Even Jamerson was using a bright tone (last cut). You can also hear Jamerson's signal clipping (the bass was recorded direct... no amps).
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