Re Jazz Bass pickups being noisy if they're not both 100%, that's because they're reverse wound/reverse polarity and function as a humbucker if both are full up. Think of a humbucker equipped guitar with a coil split switch...soon as one coil's turned off there's hum. Major noise in a high gain guitar amp but guitar players deal with it. It's probably better to think of it as a bonus that it's quiet with both PUs full up. Actually for 1960, for Fender to think of reverse wind/reverse polarity pickups was pretty slick! Re "max volume at 85% up", that's down to the taper of the pot, not the circuit or pickups.Both of those.
Jack mounted in plastic pickguard (modern P) is asking for breakage.
P pickups sound better to me "reversed".
The Jazz bass circuit is quirky and awful. Max volume at 85% up? Hum until 100% both pickups? That's not a good design at all.