trust your ears!...
Hola...well to tell if the panels are bad is very easy. They, in some parts of the panel, are dead. No sound or loss of high frequency is evident...if your Aerius panels are giving you great sound, you don't need to replace them. The time to replace is a difficult task to say. Why? Well, here in Costa Rica, we have a severe voltage fluctuations during all year long. Also our climate varies from 60% humidity to 100% in one day!!!...I think that these factors made us to had a severe panel´s epidemic some time ago...I would say about seven to eight years ago to the past. With the new panel´s materials and the new panel´s coat, the panel´s failure has being reduced dramatically, but still have some bad ones once in a while. You never know when this is going to happen, but I can say that sometimes I have customers that live far away, and both got bad panels the same day!...this is strange, right?...anyway, I think that you should keep yours at a point were you feel and listen a big change in the way that they sound...trust you ears!, the panel should porduce sound from top to bottom and from left to right, all the panel at the same time...if you have some portions of the panel that you can´t say it is sounding, then you should replace them...(please forgive my spelling)
Hope this can help,