mshan How long do high quality speakers "last"First off you have, one of the nicest sounding semi bookshelf's in the Response 2. And they still demand good money even today.
Foam roll surrounds are the best, but do rot and should only be replaced with exact same ones. As the "compliance" of the roll surround is part of the Thiel & Small parameters that has dictated your box and port size to that base driver.
I would also every 5 or so years, rotate the bass driver 180 degrees, as the cone/roll surround will sag unevenly and could eventually cause what's called "polling", this is where the voice coil is no longer in exact center of the magnet gap and starts to touch/scrape it.
Cheers George