You can't replace single drivers willy-nilly. There is a lot that goes into matching the drivers, crossover and cabinet. Also, tweeters don't have that much output, relatively.
I suggest an experiment. Sit with the speakers near the center of the room, with your seat about 2' away and listen. If you like the sound much better then you have a room acoustics issue. If they still sound dull, maybe it's just time for an upgrade?
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