Jason's sound on the black album is about as complex as rocket science man IF you wanna nail it right on 100%. He and Bob Rock went through all sorts of different bass and guitar amp's and configurations of both in order to achieve it. Notice each song sort of has an unique bass sound to them, although they all have the "Newsted trademark". But he's been using Ampeg SVT's ever since he could afford them, so I bet you wouldn't go wrong with one of those.
It's one hell of a tone, eargasm coming right up:
Sandman:
http://www.youtube.com/watch?v=rA1QpYysLhc
'Roam:
http://www.youtube.com/watch?v=X5x4UshgDfA
And Nothing Else Matters (notice how even though it's very different, it still sounds like "his tone"):
http://www.youtube.com/watch?v=7qhONSUH3kY