Alex is quite correct. Many 6000's exhibit degradation of the rubber surrounds at 10 years plus. I have replaced several.
Might as well replace all 4. Cost is around £30 per driver plus labour. Tweeters should be checked at the same time as they are prone to fail with a burnt out voice coil. Their cost is about the same but the speakers are then as good as new.
Might need some running in!!!