SB Acoustics SPL graphs systematic errors?

I had a look through a dozen driver datasheets and at the response plots for two of their kits and saw no evidence of the peak you refer to. Some of the drivers actually showed a dip in that region.

Do you have some datasheets to share that show the problem you are referring to?
 
Looks like cone breakup. You'll notice the other axes don't follow the peaks quite the same. There's more than just cone breakup happening in this region but it's sufficient to see it's why woofers are normally crossed before this, and fullrange drivers need to be carefully made.
 
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