I had a tweeter just stop working on one of mine after 10 years - the light still worked and the woofer, but no high end.

I just replaced them when I couldn't get a new pair - they actually have a very big dip in the upper mid-range which you dont realize until you compare them to a more evenly voiced speaker. I loved the way they sounded, but you might want to use this as a chance to upgrade to something more accurate. (I have krk vxt8's now which have much better freq response, but less of a sense of depth than the 20/20's - a better trade of for mixing IMO)

Using the one that still works for a little echo chamber.