We're going to conduct a more extensive research project for the old vs. new thing at some point, but my contention long before I ever came here was that the old vs. new is just as much about tiny differences from one pickup to the next as it is about age. Now that I'm here, I can tell you that we don't do anything differently, and we work extremely hard on consistency, to the point we simply reject things that are out of spec. Most companies don't have that kind of leverage, or that kind of volume that they can actually understand and quantify variables like we do.
But for the sake of the story, I recently came across an old JBJ, and we pulled it apart and analyzed it. First, the long legs and maple spacer. Do they make a difference? Maybe but no one should really hear it. So if you order a new JB with long legs, & maple spacer you negate that issue. Next, the coils were absolutely in spec, and no different. But then there was the magnet. This pickup was likely in a drawer bouncing around with other pickups, and it was plenty degaussed. It was also degaussed in an awkward pattern. Like the treble strings had a sharp drop off, and there was somewhat of a "sawtooth" strong/weak pattern across other axes. This would account for a "warmer treble" and perhaps even a smoother bass, depending on who's ears were on it, and what guitar. I regaussed the magnet, and it still has some deficiencies. Certain areas won't hold full charge. But I put that magnet back in anyway as part of the test. I haven't installed and played it yet, but I will soon.
I had a JB back in the day, and I specifically remember mine having a hot upper midrange crunch which, based on what I know today, would be attributed to a full strength Alnico V magnet. The problem is, I remember it with 1980's gear and 20 years + of nostalgia. I have since bought back a lot of my old gear and am looking forward to putting in that JB and seeing how close I get to recreating memories...