CELESTION SL6 Si - woofer replacement/repair

If you really love the speakers, you will probably need to buy another used pair, and cannibalize them for the woofers. As you probably already know, Celestion can't provide replacements, and the rubber surround was a special and integral part of the driver design. I have owned 5 versions of Celestion speakers using this woofer over the years (I still own 3 pair), and for some unknown reason, the rubber surround, while feeling soft, and otherwise looking good, can develop small fissures that almost look like they were cut with a razor knife. The speaker will still play, and doesn't sound any worse as far I as I could tell, but if it got worse over time, it would have to affect performance. I don't know if this is the problem that you are having. Anyway, it hapened to one pair that I had, and that was after I sold them to a friend. He uses them as is. It happened to another driver in one of my other pairs, but at that time, I was still able to buy a replacement.
Best thing that you can do is try to find a pair that doesn't have surround problems, and swap the driver out. They are great little speakers aren't they?

Yes - they really are great speakers, though not my primary ones at this point. I think you're right about having to keep my eye out for an old set for sale to cannibalize. I suppose it's not too far-fetched to imagine someone ending up with just one speaker for some reason and looking to unload that. But I probably shouldn't hold my breath.

I didn't really think you would find one speaker, I was talking about buying a pair. They come up cheap enough to make it worthwhile if you really like them.
They aren't my primary speakers either, but they just have really great balance of performance virtues that makes music sound good, and makes you forget what they can't do.