Why not contact the company with that question? You could ask whether this would "color" the midrange, or whether the speakers would instead be expected to pass muster with flying "colors". :)
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I contacted Avalon Acoustics and that is what they told me
These marks are usually caused by ferril fluid being deposited on the back of the ceramic material, during use. As long as the drivers are still functioning normally there is nothing to worry about.
In fact the manufacturer of this driver has been talking about changing the color of the driver, as these kind of marks often lead people to think they are defective, where in most cases these marks are nothing more than a sign of use.
Thank you guys!