[moderator's edit: as noted by Joey in a message several posts below this one, and noticed by others before he realized it, this (otherwise excellent and helpful) post has in a few places used the word relief in a way that is opposite its meaning; a common error that is easy to make when talking about tightening and loosening the truss rod nuts]
If I'm understanding this correctly, your truss rod is completely loose and the neck is higher in the middle than at either end . . . . yes, that's what's going to happen. Yes, it's completely normal to loosen the truss rods and have less and less relief! You're going backwards !
We don't loosen the truss rod nuts to get more relief: We tighten them to introduce progressively more relief (banana shape) into the neck. The more you loosen them, the more and more unplayable it will become as the neck becomes flatter and even becomes up-bowed (higher in the middle than both ends, or what you've got now).
My first observation would be that these axes DON'T like really light guage strings. We always get these questions with the string sets in the 40-90 range. Remember, you've got a 24 fret length of .25 ebony on top of hardwood laminations: It's begining to make me think that this just isn't enough string tension in total to pull this assembly into relief sometimes.
In the real world, few basses will play with a dead straight neck, although some guys can do it. But we shoot for a straight neck, and then put enough relief back with the truss rods (the 'banana shape'!?!?) to where we can play comfortably. When you get to the point where you can hear some string rattle unplugged but not any through the pickups when you're plugged in, that's about as low an action as you're going to get that's useable. Guys with a lighter touch usually fare better than guys that just wail on it with a big pick. That's just the way it is.
I'm guessing that this bass has been left long enough with dead loose truss rods that it now has an up-bow: In other words, the 'banana shape' is pointing up at the strings instead of down at the back of the neck, where it's supposed to be. You say that tuned up, it's basically rattling from the first few frets all the way to the last few frets. That's exactly what it would do in this condition.
SO . . . . we need to get some relief back in this neck.
-Tune to pitch.
-Tighten the truss rods gradually (1/4 turn at a time, please) until you can slip a medium pick (or a credit card) between the strings and the 10th fret (7th fret is not close enough to the middle). Do this with a capo on the first fret, your right hand holding the string down at the 24th fret, and hold the medium pick with your left hand. DO NOT do this with the bass laying on its back on a table: DO THIS with the bass in your lap as if you were playing sitting down. Do the E first, than double-check on the G side. The pick should fit easily between the string and fret; it can move the string ever so slightly, but if it really forces it up, add a little more relief, or let some off if the gap is too big.
-Once you're there, set your string heights across the 24th fret at 1/8 on the E and the G side.
-It will not move all at once. It may take a day or two for it to finish taking its new contour.
This should get you going. Once you've settled the bass to this, then fine-tune to where it's just right for you.
Just remember: Tightening the truss rods adds relief, not loosening them.
I'd go to a .45 set, too.
Here's a great quick study on neck set-up by the great Ibanez fretless player, Gary Willis. While aimed at his signature Ibanez axe, most of it applies to fretted basses as well:
http://www.garywillis.com/pages/bass/bassmanual/setupmanual.html Best of Luck,
J o e y
(Message edited by davehouck on October 12, 2012)
