I was playing some fairly dynamic tracks at fairly loud levels (I hit 107dB at listening position about 3m from the speakers). During one of the crescendos of the track Stimela (Coal Train) with Hugh Masekela I heard some distortion noise and then the highs went away completely. If the speakers have a sensitivity of 89dB/1W as the label says and the power handling is 70 watts I must have been using a bit more than that, considering I would need ~63 watts to play 107dB at one meter distance and I was measuring the SPL about 3m away. And since only one tweeter was playing I guess it was being loaded unproperly and therefore couldn't handle as much either.
Well, that's the first time in my life I've blown a tweeter. Interesting.