Have you checked the neck for straightness or done ay kind of fret levelling/dressing? So far all of my pitbull kits have required at least some fret work to get a nice action and remove buzz.

You can make up a straight edge from a bit of aluminium flat bar. Mark up the fret locations and file a groove so that it can sit over the frets on your fret board. Similar to this (this is for guitars, but the principle is the same for the bass):




This will tell you if the neck has any back or forwards bow.

Make sure all the frets are properly seated, a tap with a soft mallet is usually enough to get them in properly.

Then you may need to look at doing a level and dress of the frets. This is easier if you have a radius block and fret file, but can be done with some basic tools if you are careful and patient. Some good discussion here:

http://www.buildyourownguitar.com.au...ead.php?t=1775