I would not just add a tweeter to a another set of speakers that already play the high notes..
You need to have some crossovers to cut the high frequencies from the existing speaker, and cut the low frequencies from the tweeter so they don't burn up.
High frequencies will be too loud and overpowering if both speakers are playing them.
you also get some cancellation and lobing if you do not.
You should probably look into a complete component package like the above, or if your a little bit more technical, you can find some nice raw drivers here.. http://www.parts-express.com/
or here http://www.madisound.com/
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