the thicker the better! (that's what she said!)
I really like the old tele bass neck. it made a p-bass neck look like a jazz bass neck. I like a chunky thick neck because it just feels better. can't really explain it, but a thin neck just doesn't feel right. it may have to do with my hand size, but thicker necks play better for me and they have massive tone/sustain. the Warmoth p-bass neck is actually thicker than the Fender P-Bass neck, and I was really glad, too. it's kind of in the middle of a p-bass and a tele bass neck, iirc. I went from an Ibanez GSR190 (which has a jazz style neck), so my hands are cramping a little bit when playing chords, but I'll just have to build up a tolerance. j bass necks, and other thin necks, are for pansies. *flame suit on*