(i havent read all the posts on this thread) but can state=buy a beat up beater bass and work on it,tear it completly down and then clean and reassemble and make it play low,fast and without any fret buzz(of course you'll want to visit here for tips tricks and all that)
you'll then be ready and willing to attack any problem on any bass(unless of course a `warrenty' is at work,don't want to void that) but necks do change with humidity and dif sets of strings will have dif. tensions,,,
glad to see you love your instrument and are willing to buy a new neck for her
and if you want to sell your old(fretted jazz bass?) neck=i may be interested,,i have a couple open guitar stands and am considering a fretted J to compliment my fretless one,,anyway,i'm in a hurry PM me if you have a neck for sale( and enjoy the ride