You've probably already tried this, but just in case...swap the speakers (left to right and right to left) and see if the quiet tweeter moves with the swap or if it is related to the amp. From there, I would swap the cables in the same manner. From there, I would reverse polarity. If nothing helps or if the same speaker stays quiet on top, I'd send it in for service. If it is out of warranty, I would swap tweeters in the boxes...but only if my warranty was out.
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