[crackle]

I think we need to establish if the cone is moving in and out freely with no rubbing and if the inner suspension is attached. The inner suspension cant really be checked without taking the speaker out of the cabinet and looking from the side of the speaker at the inner end of the diaphragm cone.
Move the diaphragm in and out to its full extension and see if the inner suspension comes away from the chassis.
At the same time listening for any scratching or rubbing noise.
I dont know the construction of these speaker drivers but I have seen a voice coil become loose on the coil former and slide on it, that would produce the type of noise you describe.
I am not too sure how you would test for that.

But if the speaker is OK then the problem is probably in the amplifier. Possible causes dirty/worn controls, dry joints, leaky/breaking down transistors or other components.

Mike