I've been working on my Partscaster - adding a new neck and shimming the neck pocket. Usually the area underneath the neck gets shimmed, which I did initially. Before I installed the new neck, I added shims to either side to tighten up the connection between the neck and the body.
It made such a difference! It has less ice picky highs and more midrange. Some of that might be the neck, but my guess is that it's the neck pocket.