Can you turn up the bias, and do your hear the crackles? If not, likely the bias circuit went and needs to be replaced.
Switching backplates is dead on for a suggestion. If it follows the backplate, you know what went wrong.
I have a pair of M645s that are coming up on two years old, and this concerns me every time I use the speakers.
Frustrating that a speaker company that is four decades old and charges this much has this problem.
I’ve heard this happen to other people.
I can buy a pair of MLS from Best Buy that last 10 years.
Two years for a backplate is pretty poor.
Please let us know what you find out.