I have owned a pair of Advent speakers (admittedly not their subwoofer) since the early 70's.
Moving to a drier climate at 20+ years old, the foam surrounds for both woofers started deteriorating fairly severely. My memory is that I bought a Foam Speaker (pair) Repair Kit for my Advent model from
http://www.simplyspeakers.com for $25-30 + shipping. The instructions provided were straightforward enough that I completed the replacement myself in an hour or two. Ever since, I have been enjoying listening to them, even this morning.
The website now has numerous YouTube Speaker Repair videos, such as
https://www.youtube.com/watch?v=P3aBCRppbg0, to give you a feel for their surround repair/replacement process.
Also, there is a 12" repair Kit: FSK-12F, but it is likely intended for the Large Advent Loudspeakers. Consider using the Simply Speaker website's Contact Form to check FSK-12F compatibility with your Advent Sunfire 12" subwoofer.
Mark