I can't say I know a lot about big amps/speakers etc, but I got some distortion on my combo at home. I solved it by getting into the speaker cone and gently cleaning all round the cone with a vacuum cleaner. Distortion can be caused by anything that stops the cone from sliding in and out from between the magnets - like dust etc.
Besides, if all you've really done with your speakers of late is not out of the ordinary - no damp, physical damage, significant blasting at high volume, etc. What, other than dirt can have caused the damage?