[quote name='Norris' timestamp='1328187612' post='1523231']
If you want to stiffen it up a bit, either use a less flexible string (as AceBassMusic suggests with hex cores - never tried them myself), or a heavier gauge so you can get more tension on it.
The pitch of the string depends on it's length, mass and tension. The length between nut and bridge is fixed, it still needs to be tuned to 'B', so the only other variable is mass i.e. the thickness of the string. A heavier gauge string needs more tension on it to get to the same pitch - which should help with your problem with floppiness (snigger).
Any other gubbins you stick on your bass (like a different bridge, string trees, spacers, etc.) are not going to make any difference unless they [s]shorten[/s] lengthen the scale of your bass - and then your frets would be in the wrong place.
Edit: Longer string = more tension for same pitch - oops!
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+1 to this
You can't change the length of the string as the goal is the same note, but with increased tension. You can't simply increase the tension because that would change the note so the only thing you can really do is use a heavier gauge string in order to incerease the mass.
I've had a Fender Mex 5 in the past and the B always sounded completely different to the other four strings. I found that a .135 gauge string felt okay physically though.