A few months ago I modified a pair of incredibly inexpensive JBL N28 bookshelf speakers. I replaced the cap's, (over-sized Dayton metalized), resistors (dayton), internal wiring (Ixos gamma 6007), and acousta-stuff. The results made for a powerfull speaker that seems to defeat anything I throw at it for PC use.
These simple, relatively inexpensive mods *about 30 bucks* really helped make this "ok" speaker sound pretty damned nice. The metal tweeter is incredibly smooth, non fatiguing now. The bass is incredibly controlled, much more detailed. A very capable speaker and fun project!