The bass and treble are before the preamp, so they effect it about the same amount on or off. The preamp does preserve more of the highs though, so the treble cut will sound a bit more dramatic.
Since my bass was right there and plugged in I made a super quick clip to give you an idea of what it should sound like (though an '83 and a '08 tribute are probably not going to sound identical): http://www.idlecreations.com/loz/gnl_treble.mp3
I start in passive series both pickups, with all three knobs turned all the way up. Play the low E a bit and turn the treble all the way down, play and then back up. Then I flick the preamp on and into treble boost, and do the same thing. My treble knob is slightly scratchy so you can just about hear when I turn it.