I would like to say that I recently discovered all too well that not only do butyl rubber surrounds break down, they do so with very little in the way of being obvious. While there are those who say that it will not, I am on the other side of the fence. Nor do I agree with those who list 10 - 12 years as the replacement time.
The Seas P21 Excel midwoofers in my Coincident Digital Masters were totally shot. The speakers were built by Coincident in 1997. From all outward appearances, they looked PRISTINE. I mean perfect! However, once we rebuilt the drivers, the butyl surrounds were in such bad condition that I can not even begin to explain it to you. The entire surrounds were covered with two stages beyond microcracking. Add to this, they were totally dried out and stiff (which is why they were cracked), and the improvement in sound once the surrounds were rebuilt was utterly astonishing.
The midrange instantly became totally natural. The punch of both the midrange and bass took several steps forward. To the point where my midwoofer was now putting out better bass in my subwoofer - which only told me that I now need to do those drivers.
On a more controversial point, my drivers were rebuilt using foam surrounds. My speaker guru is of the opinion that its weight and compliance(this must be chosen carefully) add up to better sound in all ways. The driver is faster, punchier, more natural. It is simply a pleasure to hear. The sound just plain opens up. Of course, after perhaps 8 years, it is time to redo it, but that doesn't seem any worse than the butyl I started with.
