I showed this post to Seymour. Rather than responding directly, he asked me to post this. These are his words:
Hi Evan, here's my info...I designed the JB for several reasons...1st it was to give more output in the bridge position for players using thinner gauge strings when bending...The original P.A.F. Humbucker was designed to be used with heavy 12-13 gauge high E strings and wound G. Basically the Duncan Custom was designed to be used with 10-11 gauge High E strings...I wanted to get more output and keep the tone smooth while bending and not too harsh...almost like a steel guitar pickup that has a higher DC resistance when used near the bridge...The JB keeps the sustain, harmonics and fat full output without sounding too harsh. It works especially well when using volume swells and keeps the sustain while hitting harmonics. The 2nd reason was to give Jeff the output and tone when using a Marshall head that can be pretty clean when plugged directly in. 3rd, I wanted to make something special to give Jeff a distinctive sound that he used on "Cause We've Ended As Lovers" on the "Blow by Blow" Album produced by George Martin that also got Jeff Beck a Grammy.
Best regards,
Seymour W. Duncan
So there ya have it.