Replacing my tweeters a good idea?

As has been mentioned your speaker system is the sum of it'parts.The drivers,box and crossover values were designed to work as a unit.I contemplated and did exactly what your thinking of doing.I had an average pair of small monitors that became test subjects.I disected the little guys and in the process with alot of other research learned a great deal about what constitutes a good sounding speaker.Trying to replace your tweeters involves alot more than just buying better ones.First off even if you had the specs for your existing pair don't forget that the new ones would not only have to match electrically but physically as well.Finding replacements with the same mounting flange diameter,bolt pattern ect. is the real challenge.If your determined to improve your speakers I'd go with the earlier suggestion of installing better crossover parts and again thats only if you can get a schematic with the exact values of the existing crossover parts.I have had excellent results with this approach.Generally you'll find that the major difference between an average speaker and a great one is the quality of the coils,caps and resistors.

To add to my thoughts above I forgot to mention that the easiest and most cost effective improvement is to replace the existing internal wiring with high quality wire from Cardas,Kimber,XLO ect. The upper frequencies are particularly suseptable to changes in the wire used.