John, welcome(can always use another one). If what you term "tiny" means a lack of high frequencies, none of the steps you mention are meaningful. If any "burn-in" was necessary(doubtful), 100 seconds rather than 100 hours would be sufficient. An IC or speaker cable swap certainly shouldn't be considered "usual" and would have no effect.
Have you listened to each M3 individually to see if they both exhibit the problem? Can you play high frequency test tones to see if the tweeter is functioning at all on either or both of the speakers? Possibly an internal wire has come loose.
You can be assured that when it's operating correctly the tweeter on the M3v3s is if anything slightly superior to the previous model.
