Apogee Centaur Ribbon Mid/Tweeter Replacement

How do I know when I am to replace the mid/tweeter ribbon in these Apogees? What sort of noise/distortion happens that would tell me that they are worn? I am currently hearing almost what sounds like cartridge mistracking in my left speaker. I know that this is not the case. There are just certain frequencies where there is a buzzing noise,kind of like vibrating tin foil (if that can be imagined). There is also an occasional sound like a tube going bad. It ONLY happens when music is going through the speaker. If I just listen with no music, neither noise is there. If you can be of help, please let me know. Thank you!
Hi go to www.apogespeaker.com users site press users group everything you need to know for apogee