I have seen a bad Polypropylene cap, but it was bad from the factory, I've never seen one go bad. I would suggest, you go more the way that Al suggested, get a test tone cd and a decent mic and measure frequencies at about a 3ft away... turn your volume up until you measure 90 db or so, then without changing your volume, change the frequencies and measure... I'd probably check from 2k to 5k in 500hz brackets. All should range from 87 to 93.
If it all falls in that range, your tweeters are fine. I think we all have been tricked into thinking there are problems... on the other hand, if it measures weird then at least you have a frequency that gives a barometer of where to look. Odd things happen, you could have gotten tweeters with the wrong diaphrams...
If you plan on continuing in the speaker game, I would highly recommend purchasing a LCR meter
If it all falls in that range, your tweeters are fine. I think we all have been tricked into thinking there are problems... on the other hand, if it measures weird then at least you have a frequency that gives a barometer of where to look. Odd things happen, you could have gotten tweeters with the wrong diaphrams...
If you plan on continuing in the speaker game, I would highly recommend purchasing a LCR meter